diff --git a/changelog.d/hi-credit-start-dates.fixed.md b/changelog.d/hi-credit-start-dates.fixed.md
new file mode 100644
index 00000000000..9e306adbc1b
--- /dev/null
+++ b/changelog.d/hi-credit-start-dates.fixed.md
@@ -0,0 +1 @@
+Stop paying the Hawaii earned income tax credit before 2018 and the Act 115 refund outside 2021, and use each year's Hawaii military reserve pay exclusion cap for 2015-2025.
diff --git a/policyengine_us/parameters/gov/states/hi/tax/income/credits/act_115_rebate/joint.yaml b/policyengine_us/parameters/gov/states/hi/tax/income/credits/act_115_rebate/joint.yaml
index 7b320be909d..22ae1b468b4 100644
--- a/policyengine_us/parameters/gov/states/hi/tax/income/credits/act_115_rebate/joint.yaml
+++ b/policyengine_us/parameters/gov/states/hi/tax/income/credits/act_115_rebate/joint.yaml
@@ -15,8 +15,14 @@ brackets:
   - threshold:
       2021-01-01: -.inf
     amount:
+      # Model years start in 2015. Act 115 paid a one-time refund to filers
+      # of a 2021 Form N-11.
+      2000-01-01: 0
       2021-01-01: 300
+      2022-01-01: 0
   - threshold:
       2021-01-01: 200_000
     amount:
+      2000-01-01: 0
       2021-01-01: 100
+      2022-01-01: 0
diff --git a/policyengine_us/parameters/gov/states/hi/tax/income/credits/act_115_rebate/single.yaml b/policyengine_us/parameters/gov/states/hi/tax/income/credits/act_115_rebate/single.yaml
index f002f2aff5b..089e4a1d9bc 100644
--- a/policyengine_us/parameters/gov/states/hi/tax/income/credits/act_115_rebate/single.yaml
+++ b/policyengine_us/parameters/gov/states/hi/tax/income/credits/act_115_rebate/single.yaml
@@ -15,8 +15,14 @@ brackets:
   - threshold:
       2021-01-01: -.inf
     amount:
+      # Model years start in 2015. Act 115 paid a one-time refund to filers
+      # of a 2021 Form N-11.
+      2000-01-01: 0
       2021-01-01: 300
+      2022-01-01: 0
   - threshold:
       2021-01-01: 100_000
     amount:
+      2000-01-01: 0
       2021-01-01: 100
+      2022-01-01: 0
diff --git a/policyengine_us/parameters/gov/states/hi/tax/income/credits/eitc/match.yaml b/policyengine_us/parameters/gov/states/hi/tax/income/credits/eitc/match.yaml
index fb8d3cad97b..321364fe888 100644
--- a/policyengine_us/parameters/gov/states/hi/tax/income/credits/eitc/match.yaml
+++ b/policyengine_us/parameters/gov/states/hi/tax/income/credits/eitc/match.yaml
@@ -1,6 +1,9 @@
 description: Hawaii matches this percent of the federal EITC.
 values:
-  2017-01-01: 0.2
+  # Model years start in 2015. The credit applies to taxable years beginning
+  # after December 31, 2017.
+  2000-01-01: 0
+  2018-01-01: 0.2
   2023-01-01: 0.4
   2028-01-01: 0.2
 metadata:
@@ -9,6 +12,10 @@ metadata:
   reference:
     - title: Hawaii Income Tax Law, Chapter 235, Section 235-55.75
       href: https://files.hawaii.gov/tax/legal/hrs/hrs_235.pdf#page=43
+    - title: 2018 Hawaii N-11 Instructions, What's new (Act 107, SLH 2017)
+      href: https://files.hawaii.gov/tax/forms/2018/n11ins.pdf#page=2
+    - title: Hawaii Tax Form N-356 2018
+      href: https://web.archive.org/web/20190819110547/http://files.hawaii.gov/tax/forms/2018/n356_i.pdf#page=2
     - title: Hawaii Tax Form N-356 2022
       href: https://files.hawaii.gov/tax/forms/2022/n356_i.pdf
     - title: Hawaii Tax Form N-356 2023
diff --git a/policyengine_us/parameters/gov/states/hi/tax/income/credits/non_refundable.yaml b/policyengine_us/parameters/gov/states/hi/tax/income/credits/non_refundable.yaml
index 0013368000a..e9c0f7f9ffb 100644
--- a/policyengine_us/parameters/gov/states/hi/tax/income/credits/non_refundable.yaml
+++ b/policyengine_us/parameters/gov/states/hi/tax/income/credits/non_refundable.yaml
@@ -1,6 +1,9 @@
 description: Hawaii provides the following non-refundable tax credits.
 values:
-  2021-01-01:
+  # Model years start in 2015. Hawaii had no earned income tax credit
+  # before 2018.
+  2000-01-01: []
+  2018-01-01:
     - hi_eitc
   2023-01-01: []
 
@@ -11,5 +14,9 @@ metadata:
   reference:
     - title: Hawaii Revised Statutes Title 14. Taxation 235. Income Tax Law, 235-55.75 Earned income tax credit. # repealed in 2023
       href: https://law.justia.com/codes/hawaii/2022/title-14/chapter-235/section-235-55-75/
+    - title: 2018 Hawaii N-11 Instructions, What's new (Act 107, SLH 2017)
+      href: https://files.hawaii.gov/tax/forms/2018/n11ins.pdf#page=2
+    - title: 2018 Hawaii Form N-356 Instructions
+      href: https://web.archive.org/web/20190819110547/http://files.hawaii.gov/tax/forms/2018/n356_i.pdf#page=2
     - title: 2025 Hawaii N-11 Instructions
       href: https://files.hawaii.gov/tax/forms/current/n11ins.pdf
diff --git a/policyengine_us/parameters/gov/states/hi/tax/income/credits/refundable.yaml b/policyengine_us/parameters/gov/states/hi/tax/income/credits/refundable.yaml
index 264449d0df2..f8b1648881d 100644
--- a/policyengine_us/parameters/gov/states/hi/tax/income/credits/refundable.yaml
+++ b/policyengine_us/parameters/gov/states/hi/tax/income/credits/refundable.yaml
@@ -1,5 +1,12 @@
 description: Hawaii provides the following refundable tax credits.
 values:
+  # Model years start in 2015. The Act 115 refund went only to filers of a
+  # 2021 Form N-11.
+  2000-01-01:
+    - hi_food_excise_credit
+    - hi_cdcc
+    - hi_tax_credit_for_low_income_household_renters
+
   2021-01-01:
     - hi_food_excise_credit
     - hi_cdcc
@@ -22,6 +29,10 @@ metadata:
   period: year
   label: Hawaii refundable tax credits
   reference:
+    - title: 2018 Hawaii N-11 Tax Form
+      href: https://files.hawaii.gov/tax/forms/2018/n11_i.pdf#page=3
+    - title: Hawaii Department of Taxation Announcement No. 2022-03, Act 115 Income Tax Refunds
+      href: https://files.hawaii.gov/tax/news/announce/ann22-03.pdf#page=2
     - title: 2022 Hawaii N-11 Tax Form
       href: https://files.hawaii.gov/tax/forms/2022/n11_i.pdf#page=3
     - title: 2021 Hawaii N-11 Tax Form
diff --git a/policyengine_us/parameters/gov/states/hi/tax/income/subtractions/military_pay/cap.yaml b/policyengine_us/parameters/gov/states/hi/tax/income/subtractions/military_pay/cap.yaml
index 019814a220c..bc3533ef2db 100644
--- a/policyengine_us/parameters/gov/states/hi/tax/income/subtractions/military_pay/cap.yaml
+++ b/policyengine_us/parameters/gov/states/hi/tax/income/subtractions/military_pay/cap.yaml
@@ -1,8 +1,16 @@
 description: Hawaii caps the military reserve or Hawaii national guard duty pay exclusion at this amount.
-values: 
-  2021-01-01: 7_345
+values:
+  2015-01-01: 6_198
+  2016-01-01: 6_279
+  2017-01-01: 6_410
+  2018-01-01: 6_564
+  2019-01-01: 6_735
+  2020-01-01: 6_943
+  2021-01-01: 7_152
+  2022-01-01: 7_345
   2023-01-01: 7_683
   2024-01-01: 8_082
+  2025-01-01: 8_636
 
 metadata:
   uprating: gov.irs.uprating
@@ -10,21 +18,29 @@ metadata:
   period: year
   unit: currency-USD
   reference:
-  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2022 
-    href: https://files.hawaii.gov/tax/forms/2022/n11ins.pdf#page=13
-  - title: Hawaii 2022 Individual Income Tax Return N-11 From Rev.2022
-    href: https://files.hawaii.gov/tax/forms/2022/n11_i.pdf#page=2
-  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2021 
+  # The value of the subtraction is derived from compensation for
+  # performance of duty, equivalent to pay received for forty-eight drills
+  - title: Hawaii Income Tax Law, Chapter 235, Section 235-7, (a) (7)
+    href: https://files.hawaii.gov/tax/legal/hrs/hrs_235.pdf#page=22
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2015, Line 15
+    href: https://web.archive.org/web/20160207173809/http://files.hawaii.gov/tax/forms/2015/n11ins.pdf#page=14
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2016, Line 15
+    href: https://web.archive.org/web/20170214234012/http://files.hawaii.gov/tax/forms/2016/n11ins.pdf#page=15
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2017, Line 15
+    href: https://web.archive.org/web/20180417023253/http://files.hawaii.gov/tax/forms/2017/n11ins.pdf#page=15
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2018, Line 15
+    href: https://files.hawaii.gov/tax/forms/2018/n11ins.pdf#page=15
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2019, Line 15
+    href: https://files.hawaii.gov/tax/forms/2019/n11ins.pdf#page=15
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2020, Line 15
+    href: https://files.hawaii.gov/tax/forms/2020/n11ins.pdf#page=15
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2021, Line 15
     href: https://files.hawaii.gov/tax/forms/2021/n11ins.pdf#page=13
-  - title: Hawaii 2021 Individual Income Tax Return N-11 From Rev.2021
-    href: https://files.hawaii.gov/tax/forms/2021/n11_i.pdf#page=2  
-  # The value of the subtraction is derived from compensation for  
-  # performance of duty, equivalent to pay received for forty-eight drills 
-  - title: Hawaii Income Tax Law, Chapter 235, Section 235-7, (a) (7)  
-    href: https://files.hawaii.gov/tax/legal/hrs/hrs_235.pdf#page=22 
-  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2023
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2022, Line 15
+    href: https://files.hawaii.gov/tax/forms/2022/n11ins.pdf#page=13
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2023, Line 15
     href: https://files.hawaii.gov/tax/forms/2023/n11ins.pdf#page=13
-  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2024
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2024, Line 15
     href: https://files.hawaii.gov/tax/forms/2024/n11ins.pdf#page=13
-  - title: 2025 Hawaii N-11 Instructions Military Pay Exclusion
-    href: https://files.hawaii.gov/tax/forms/current/n11ins.pdf#page=13
+  - title: Hawaii Resident Income Tax Instructions N-11 Rev.2025, Line 15
+    href: https://files.hawaii.gov/tax/forms/2025/n11ins.pdf#page=13
diff --git a/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_act_115_rebate.yaml b/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_act_115_rebate.yaml
index 13ebfd8d94a..6ca040f88c0 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_act_115_rebate.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_act_115_rebate.yaml
@@ -162,3 +162,52 @@
         state_code: HI
   output:
     hi_act_115_rebate: 0
+
+# Act 115 refunds went only to filers of a 2021 Form N-11 (DOTAX
+# Announcement 2022-03, p.2).
+- name: No refund for 2017
+  period: 2017
+  input:
+    adjusted_gross_income: 50_000
+    filing_status: SINGLE
+    state_code: HI
+  output:
+    hi_act_115_rebate: 0
+
+- name: No refund for 2020
+  period: 2020
+  input:
+    adjusted_gross_income: 50_000
+    filing_status: SINGLE
+    state_code: HI
+  output:
+    hi_act_115_rebate: 0
+
+- name: No refund for 2022
+  period: 2022
+  input:
+    adjusted_gross_income: 50_000
+    filing_status: SINGLE
+    state_code: HI
+  output:
+    hi_act_115_rebate: 0
+
+- name: No refund for joint filers for 2020
+  period: 2020
+  input:
+    people:
+      head:
+        age: 40
+      spouse:
+        age: 40
+    tax_units:
+      tax_unit:
+        members: [head, spouse]
+        adjusted_gross_income: 60_000
+        filing_status: JOINT
+    households:
+      household:
+        members: [head, spouse]
+        state_code: HI
+  output:
+    hi_act_115_rebate: 0
diff --git a/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_credit_start_dates.yaml b/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_credit_start_dates.yaml
new file mode 100644
index 00000000000..f1f3d725874
--- /dev/null
+++ b/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_credit_start_dates.yaml
@@ -0,0 +1,146 @@
+# A single filer, age 30, with $8,000 of wages in each year around the
+# credits' start dates. The federal EITC is on its plateau in each year
+# tested (single, no qualifying child), and the Hawaii food/excise credit is
+# $100 for federal AGI of $5,000 to $10,000 (Form N-311, 2017-2022).
+# Hawaii's EITC applies to taxable years beginning after December 31, 2017
+# (Act 107, SLH 2017; 2018 Form N-356 instructions, p.2). Act 115 (SLH 2022)
+# refunds went only to filers of a 2021 Form N-11 (DOTAX Announcement
+# 2022-03, p.2).
+- name: Case 1, no Hawaii EITC or Act 115 refund in 2017.
+  period: 2017
+  input:
+    people:
+      person1:
+        age: 30
+        employment_income: 8_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_code: HI
+  output:
+    eitc: 510
+    hi_eitc_potential: 0
+    hi_non_refundable_credits: 0
+    hi_act_115_rebate: 0
+    hi_food_excise_credit: 100
+    hi_refundable_credits: 100
+    # Taxable income: 8,000 - 2,200 standard deduction - 1,144 exemption
+    # = 4,656. Tax: 1.4% * 2,400 + 3.2% * 2,256 = 105.792.
+    # 105.792 - 0 nonrefundable - 100 refundable = 5.792.
+    hi_income_tax: 5.792
+  absolute_error_margin: 0.01
+
+- name: Case 2, Hawaii EITC starts in 2018 and no Act 115 refund.
+  period: 2018
+  input:
+    people:
+      person1:
+        age: 30
+        employment_income: 8_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_code: HI
+  output:
+    eitc: 519
+    # 20% * 519
+    hi_eitc_potential: 103.8
+    hi_act_115_rebate: 0
+    hi_refundable_credits: 100
+  absolute_error_margin: 0.01
+
+- name: Case 3, Hawaii EITC and no Act 115 refund in 2020.
+  period: 2020
+  input:
+    people:
+      person1:
+        age: 30
+        employment_income: 8_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_code: HI
+  output:
+    eitc: 538
+    # 20% * 538
+    hi_eitc_potential: 107.6
+    hi_act_115_rebate: 0
+    hi_refundable_credits: 100
+  absolute_error_margin: 0.01
+
+- name: Case 4, Act 115 refund for 2021.
+  period: 2021
+  input:
+    people:
+      person1:
+        age: 30
+        employment_income: 8_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_code: HI
+  output:
+    # 15.3% * 8,000 under the 2021 phase-in.
+    eitc: 1_224
+    # 20% * 1,224
+    hi_eitc_potential: 244.8
+    # One exemption * $300 for federal AGI under $100,000.
+    hi_act_115_rebate: 300
+    # 100 food/excise + 300 Act 115
+    hi_refundable_credits: 400
+  absolute_error_margin: 0.01
+
+- name: Case 5, no Act 115 refund for 2022.
+  period: 2022
+  input:
+    people:
+      person1:
+        age: 30
+        employment_income: 8_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_code: HI
+  output:
+    eitc: 560
+    # 20% * 560
+    hi_eitc_potential: 112
+    hi_act_115_rebate: 0
+    hi_refundable_credits: 100
+  absolute_error_margin: 0.01
+
+- name: Case 6, no Hawaii EITC or Act 115 refund in 2016.
+  period: 2016
+  input:
+    people:
+      person1:
+        age: 30
+        employment_income: 8_000
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_code: HI
+  output:
+    hi_eitc_potential: 0
+    hi_non_refundable_credits: 0
+    hi_act_115_rebate: 0
+    hi_refundable_credits: 100
+  absolute_error_margin: 0.01
diff --git a/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_eitc.yaml b/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_eitc.yaml
index ad2cfec1980..918a84ddd9a 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_eitc.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/credits/hi_eitc.yaml
@@ -43,3 +43,62 @@
     state_code: HI
   output:
     hi_eitc: 400
+
+# Act 107 (SLH 2017) created the credit for taxable years beginning after
+# December 31, 2017 (2018 Form N-356 instructions, p.2).
+- name: No Hawaii EITC in 2016
+  period: 2016
+  input:
+    eitc: 506
+    state_code: HI
+    hi_income_tax_before_non_refundable_credits: 1_000_000
+  output:
+    hi_eitc_potential: 0
+    hi_eitc: 0
+    hi_non_refundable_credits: 0
+
+- name: No Hawaii EITC in 2017
+  period: 2017
+  input:
+    eitc: 510
+    state_code: HI
+    hi_income_tax_before_non_refundable_credits: 1_000_000
+  output:
+    hi_eitc_potential: 0
+    hi_eitc: 0
+    hi_non_refundable_credits: 0
+
+- name: 20% nonrefundable match in 2018, the first year
+  period: 2018
+  input:
+    eitc: 519
+    state_code: HI
+    hi_income_tax_before_non_refundable_credits: 1_000_000
+  output:
+    # 20% * 519
+    hi_eitc_potential: 103.8
+    hi_eitc: 103.8
+    hi_non_refundable_credits: 103.8
+
+- name: 20% nonrefundable match in 2020
+  period: 2020
+  input:
+    eitc: 538
+    state_code: HI
+    hi_income_tax_before_non_refundable_credits: 1_000_000
+  output:
+    # 20% * 538
+    hi_eitc_potential: 107.6
+    hi_eitc: 107.6
+    hi_non_refundable_credits: 107.6
+
+- name: 20% nonrefundable match in 2021
+  period: 2021
+  input:
+    eitc: 1_000
+    state_code: HI
+    hi_income_tax_before_non_refundable_credits: 1_000_000
+  output:
+    hi_eitc_potential: 200
+    hi_eitc: 200
+    hi_non_refundable_credits: 200
diff --git a/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/subtractions/hi_military_pay_exclusion.yaml b/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/subtractions/hi_military_pay_exclusion.yaml
index 8da03d6eab8..413c9a24b50 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/subtractions/hi_military_pay_exclusion.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/hi/tax/income/subtractions/hi_military_pay_exclusion.yaml
@@ -9,9 +9,55 @@
 
 - name: Military reserve or national guard duty pay exceeds the max amount
   period: 2021
+  input:
+    military_service_income: 8_000
+    is_tax_unit_head_or_spouse: true
+    state_code: HI
+  output:
+    # 2021 N-11 instructions, line 15: $7,152.
+    hi_military_pay_exclusion: 7_152
+
+- name: The 2015 cap is $6,198
+  period: 2015
+  input:
+    military_service_income: 8_000
+    is_tax_unit_head_or_spouse: true
+    state_code: HI
+  output:
+    hi_military_pay_exclusion: 6_198
+
+- name: The 2018 cap is $6,564
+  period: 2018
+  input:
+    military_service_income: 8_000
+    is_tax_unit_head_or_spouse: true
+    state_code: HI
+  output:
+    hi_military_pay_exclusion: 6_564
+
+- name: The 2020 cap is $6,943
+  period: 2020
+  input:
+    military_service_income: 8_000
+    is_tax_unit_head_or_spouse: true
+    state_code: HI
+  output:
+    hi_military_pay_exclusion: 6_943
+
+- name: The 2022 cap is $7,345
+  period: 2022
   input:
     military_service_income: 8_000
     is_tax_unit_head_or_spouse: true
     state_code: HI
   output:
     hi_military_pay_exclusion: 7_345
+
+- name: The 2025 cap is $8,636
+  period: 2025
+  input:
+    military_service_income: 9_000
+    is_tax_unit_head_or_spouse: true
+    state_code: HI
+  output:
+    hi_military_pay_exclusion: 8_636
diff --git a/policyengine_us/tests/unit/test_hi_credit_start_dates_properties.py b/policyengine_us/tests/unit/test_hi_credit_start_dates_properties.py
new file mode 100644
index 00000000000..a87bb838c00
--- /dev/null
+++ b/policyengine_us/tests/unit/test_hi_credit_start_dates_properties.py
@@ -0,0 +1,236 @@
+"""Properties of the years in which Hawaii's EITC and Act 115 refund apply.
+
+``backdate_parameters`` copies each parameter's first value back to 2015, so
+these credits paid in years before they existed until their parameters were
+dated. The law:
+
+- Hawaii's earned income tax credit applies to taxable years beginning after
+  December 31, 2017 (Act 107, SLH 2017; 2018 Form N-356 instructions, p.2):
+  20% of the federal credit, nonrefundable, for 2018-2022, then 40% and
+  refundable from 2023 (Act 163, SLH 2023).
+- Act 115 (SLH 2022) refunds went only to filers of a 2021 Form N-11: $300
+  per exemption under the federal AGI threshold ($100,000; $200,000 for joint
+  filers and surviving spouses) and $100 at or above it (DOTAX Announcement
+  2022-03, p.2).
+- The military reserve or Hawaii National Guard duty pay exclusion is capped
+  at each year's N-11 instruction amount (line 15).
+
+For every grid point, and for Hypothesis-generated batches, in 2015-2026:
+
+- ``hi_act_115_rebate`` is zero outside 2021 and equals the announcement's
+  table times the number of people on the return in 2021;
+- ``hi_eitc_potential`` is the year's match rate times the federal EITC, so
+  it is zero before 2018;
+- ``hi_non_refundable_credits`` is zero before 2018 and from 2023, and never
+  exceeds the Hawaii EITC or the tax it offsets in 2018-2022;
+- refundable credits beyond the food/excise, dependent care and renters'
+  credits are the Act 115 refund (2021 only) and the Hawaii EITC (from 2023);
+- before 2018, Hawaii income tax is the tax before credits minus the
+  refundable credits, since no nonrefundable credit applies.
+"""
+
+import itertools
+
+import numpy as np
+import pytest
+from hypothesis import given, settings, strategies as st
+
+from policyengine_us import Simulation
+
+YEARS = list(range(2015, 2027))
+
+# Hawaii EITC match rate by year (HRS 235-55.75, Act 107 SLH 2017, Act 163
+# SLH 2023).
+EITC_MATCH = {
+    year: 0.0 if year < 2018 else 0.2 if year < 2023 else 0.4 for year in YEARS
+}
+ACT_115_YEAR = 2021
+ACT_115_HIGH, ACT_115_LOW = 300, 100
+JOINT_STATUSES = {"JOINT", "SURVIVING_SPOUSE"}
+ACT_115_THRESHOLD = {"joint": 200_000, "other": 100_000}
+
+# Military reserve or Hawaii National Guard duty pay exclusion cap, from each
+# year's N-11 instructions, line 15.
+MILITARY_PAY_CAP = {
+    2015: 6_198,
+    2016: 6_279,
+    2017: 6_410,
+    2018: 6_564,
+    2019: 6_735,
+    2020: 6_943,
+    2021: 7_152,
+    2022: 7_345,
+    2023: 7_683,
+    2024: 8_082,
+    2025: 8_636,
+}
+
+# (adults, children): single, head of household and joint returns.
+HOUSEHOLDS = [(1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]
+WAGES = [0, 4_000, 8_000, 15_000, 40_000, 99_999, 100_000, 150_000, 200_000]
+AGES = [30, 70]
+GRID = list(itertools.product(HOUSEHOLDS, WAGES, AGES))
+
+
+def every_year(value):
+    return {year: value for year in YEARS}
+
+
+def hawaii_simulation(points):
+    """One Hawaii tax unit per (household, wages, age) point."""
+    people, tax_units, households = {}, {}, {}
+    for i, ((adults, children), wages, age) in enumerate(points):
+        members = []
+        for a in range(adults):
+            name = f"u{i}_adult{a}"
+            people[name] = {
+                "age": every_year(age),
+                "employment_income": every_year(wages if a == 0 else 0),
+            }
+            members.append(name)
+        for c in range(children):
+            name = f"u{i}_child{c}"
+            people[name] = {"age": every_year(8 + c)}
+            members.append(name)
+        tax_units[f"u{i}"] = {"members": members}
+        households[f"u{i}"] = {"members": members, "state_code": every_year("HI")}
+    return Simulation(
+        situation={"people": people, "tax_units": tax_units, "households": households}
+    )
+
+
+def act_115_reference(sim, year):
+    """DOTAX Announcement 2022-03 table times exemptions, for 2021 only."""
+    if year != ACT_115_YEAR:
+        return np.zeros(len(sim.calculate("tax_unit_id", year)))
+    statuses = sim.calculate("filing_status", year).decode_to_str()
+    agi = sim.calculate("adjusted_gross_income", year)
+    threshold = np.where(
+        np.isin(statuses, list(JOINT_STATUSES)),
+        ACT_115_THRESHOLD["joint"],
+        ACT_115_THRESHOLD["other"],
+    )
+    per_exemption = np.where(agi < threshold, ACT_115_HIGH, ACT_115_LOW)
+    people = sim.calculate("tax_unit_size", year)
+    return per_exemption * people
+
+
+def check_invariants(sim, year):
+    calc = lambda variable: sim.calculate(variable, year)  # noqa: E731
+
+    rebate = calc("hi_act_115_rebate")
+    np.testing.assert_allclose(rebate, act_115_reference(sim, year), atol=0.01)
+
+    eitc_potential = calc("hi_eitc_potential")
+    np.testing.assert_allclose(
+        eitc_potential, EITC_MATCH[year] * calc("eitc"), atol=0.01
+    )
+
+    non_refundable = calc("hi_non_refundable_credits")
+    tax_before_credits = calc("hi_income_tax_before_non_refundable_credits")
+    if year < 2018 or year >= 2023:
+        assert np.all(non_refundable == 0)
+    else:
+        assert np.all(non_refundable >= 0)
+        assert np.all(
+            non_refundable <= np.minimum(eitc_potential, tax_before_credits) + 0.01
+        )
+
+    base_refundable = (
+        calc("hi_food_excise_credit")
+        + calc("hi_cdcc")
+        + calc("hi_tax_credit_for_low_income_household_renters")
+    )
+    expected_extra = rebate if year == ACT_115_YEAR else 0
+    if year >= 2023:
+        expected_extra = expected_extra + calc("hi_eitc")
+    np.testing.assert_allclose(
+        calc("hi_refundable_credits") - base_refundable, expected_extra, atol=0.01
+    )
+
+    if year < 2018:
+        np.testing.assert_allclose(
+            calc("hi_income_tax"),
+            tax_before_credits - calc("hi_refundable_credits"),
+            atol=0.01,
+        )
+
+
+@pytest.fixture(scope="module")
+def grid_sim():
+    return hawaii_simulation(GRID)
+
+
+def test_grid_premises(grid_sim):
+    # The grid reaches every filing status the Act 115 table distinguishes
+    # here, both sides of each AGI threshold, and a positive federal EITC.
+    statuses = set(grid_sim.calculate("filing_status", 2021).decode_to_str())
+    assert {"SINGLE", "HEAD_OF_HOUSEHOLD", "JOINT"} <= statuses
+    agi = grid_sim.calculate("adjusted_gross_income", 2021)
+    assert (agi < 100_000).any() and (agi >= 200_000).any()
+    for year in YEARS:
+        assert (grid_sim.calculate("eitc", year) > 0).any()
+
+
+@pytest.mark.parametrize("year", YEARS)
+def test_grid_invariants(grid_sim, year):
+    check_invariants(grid_sim, year)
+
+
+def test_act_115_refund_is_paid_only_for_2021(grid_sim):
+    paid = {year: grid_sim.calculate("hi_act_115_rebate", year).sum() for year in YEARS}
+    assert paid[ACT_115_YEAR] > 0
+    assert all(paid[year] == 0 for year in YEARS if year != ACT_115_YEAR)
+
+
+def test_hawaii_eitc_starts_in_2018(grid_sim):
+    for year in YEARS:
+        paid = grid_sim.calculate("hi_eitc_potential", year).sum()
+        assert (paid > 0) == (year >= 2018), year
+
+
+GENERATED_POINTS = st.lists(
+    st.tuples(
+        st.sampled_from(HOUSEHOLDS),
+        st.integers(min_value=0, max_value=300_000),
+        st.sampled_from(AGES),
+    ),
+    min_size=1,
+    max_size=8,
+)
+
+
+@settings(max_examples=8, deadline=None)
+@given(points=GENERATED_POINTS, year=st.sampled_from(YEARS))
+def test_generated_invariants(points, year):
+    # Witnesses on each side of the Act 115 thresholds and on the EITC
+    # plateau keep every batch from passing vacuously.
+    witnesses = [((1, 0), 8_000, 30), ((2, 1), 150_000, 30), ((2, 0), 250_000, 70)]
+    check_invariants(hawaii_simulation(points + witnesses), year)
+
+
+def test_military_pay_exclusion_matches_published_caps():
+    """Differential test: the exclusion against each year's N-11 cap."""
+    pays = [0, 3_000, 6_000, 7_000, 8_000, 8_636, 9_000, 20_000]
+    people, tax_units, households = {}, {}, {}
+    for i, pay in enumerate(pays):
+        name = f"p{i}"
+        people[name] = {
+            "age": {year: 40 for year in MILITARY_PAY_CAP},
+            "military_service_income": {year: pay for year in MILITARY_PAY_CAP},
+        }
+        tax_units[name] = {"members": [name]}
+        households[name] = {
+            "members": [name],
+            "state_code": {year: "HI" for year in MILITARY_PAY_CAP},
+        }
+    sim = Simulation(
+        situation={"people": people, "tax_units": tax_units, "households": households}
+    )
+    for year, cap in MILITARY_PAY_CAP.items():
+        np.testing.assert_allclose(
+            sim.calculate("hi_military_pay_exclusion", year),
+            np.minimum(pays, cap),
+            atol=0.01,
+            err_msg=str(year),
+        )