diff --git a/TrayApp.Test/CommandLineParserTests.cs b/TrayApp.Test/CommandLineParserTests.cs
index c685c21..b2b05cc 100644
--- a/TrayApp.Test/CommandLineParserTests.cs
+++ b/TrayApp.Test/CommandLineParserTests.cs
@@ -68,9 +68,18 @@ public void TryParse_WithAnUnreadableDuration_Fails()
 	}
 
 	[TestMethod]
-	public void TryParse_WithADurationLongerThanATimerCanWait_FailsWithAUsageError()
+	public void TryParse_WithADurationUnderATick_Fails()
 	{
-		Assert.IsFalse(CommandLineParser.TryParse(["--for", "25d"], NoOptions, out _, out string error));
+		Assert.IsFalse(CommandLineParser.TryParse(["--for", "0.00000001s"], NoOptions, out _, out string error));
+		Assert.Contains("is not a duration", error, StringComparison.Ordinal);
+	}
+
+	[TestMethod]
+	[DataRow("25d")]
+	[DataRow("20000000d")]
+	public void TryParse_WithADurationLongerThanATimerCanWait_FailsWithAUsageError(string text)
+	{
+		Assert.IsFalse(CommandLineParser.TryParse(["--for", text], NoOptions, out _, out string error));
 		Assert.Contains("--for can be at most 24 days", error, StringComparison.Ordinal);
 	}
 
diff --git a/TrayApp.Test/DurationParserTests.cs b/TrayApp.Test/DurationParserTests.cs
index f584e9a..5357f0d 100644
--- a/TrayApp.Test/DurationParserTests.cs
+++ b/TrayApp.Test/DurationParserTests.cs
@@ -64,6 +64,8 @@ public void TryParse_WithABareNumberAfterAUnit_ReturnsFalse()
 	[DataRow("5x")]
 	[DataRow("m5")]
 	[DataRow("1e9h")]
+	[DataRow("0.00000001s")]
+	[DataRow("0.00000004s")]
 	public void TryParse_WithAnInvalidDuration_ReturnsFalse(string? text)
 	{
 		Assert.IsFalse(DurationParser.TryParse(text, out TimeSpan duration));
@@ -81,6 +83,8 @@ public void TryParse_AtTheLongestWaitATimerTakes_ReturnsTheDuration()
 	[TestMethod]
 	[DataRow("25d")]
 	[DataRow("24d21h")]
+	[DataRow("20000000d")]
+	[DataRow("99999999999999999999999999999999999999999999999999999999999999999999h")]
 	public void TryParse_LongerThanATimerCanWait_ReturnsFalse(string text)
 	{
 		// ManualResetEventSlim.Wait and the tray's timer both throw past int.MaxValue milliseconds, about
@@ -89,6 +93,26 @@ public void TryParse_LongerThanATimerCanWait_ReturnsFalse(string text)
 		Assert.AreEqual(TimeSpan.Zero, duration);
 	}
 
+	[TestMethod]
+	[DataRow("0.00000001s")]
+	[DataRow("0.00000004s")]
+	public void TryParseAnyLength_WithADurationThatRoundsToZero_ReturnsFalse(string text)
+	{
+		// The value is positive, but under one tick, so it would be a zero-length run.
+		Assert.IsFalse(DurationParser.TryParseAnyLength(text, out TimeSpan duration));
+		Assert.AreEqual(TimeSpan.Zero, duration);
+	}
+
+	[TestMethod]
+	[DataRow("20000000d")]
+	[DataRow("99999999999999999999999999999999999999999999999999999999999999999999h")]
+	public void TryParseAnyLength_WithADurationBeyondTimeSpan_ReturnsTheLongestTimeSpan(string text)
+	{
+		// It is a duration, just one too long to hold, so the caller reports it as too long.
+		Assert.IsTrue(DurationParser.TryParseAnyLength(text, out TimeSpan duration));
+		Assert.AreEqual(TimeSpan.MaxValue, duration);
+	}
+
 	[TestMethod]
 	public void MaxDuration_IsTheLongestTimeoutAWaitTakes()
 	{
diff --git a/TrayApp/Cli/DurationParser.cs b/TrayApp/Cli/DurationParser.cs
index 531bf9b..e5824bc 100644
--- a/TrayApp/Cli/DurationParser.cs
+++ b/TrayApp/Cli/DurationParser.cs
@@ -52,7 +52,10 @@ public static bool TryParse(string? text, out TimeSpan duration)
 	/// </summary>
 	/// <param name="text">The text to read.</param>
 	/// <param name="duration">Receives the duration when parsing succeeds.</param>
-	/// <returns><see langword="true"/> when <paramref name="text"/> is a positive duration.</returns>
+	/// <returns>
+	/// <see langword="true"/> when <paramref name="text"/> is a duration of at least one tick. A duration too long
+	/// for <see cref="TimeSpan"/> to hold is returned as <see cref="TimeSpan.MaxValue"/>.
+	/// </returns>
 	internal static bool TryParseAnyLength(string? text, out TimeSpan duration)
 	{
 		duration = TimeSpan.Zero;
@@ -78,12 +81,29 @@ internal static bool TryParseAnyLength(string? text, out TimeSpan duration)
 			sawAnyPart = true;
 		}
 
-		if (!sawAnyPart || totalSeconds <= 0 || totalSeconds > TimeSpan.MaxValue.TotalSeconds)
+		if (!sawAnyPart || totalSeconds <= 0)
+		{
+			return false;
+		}
+
+		// A value TimeSpan cannot hold is still a duration, just one too long for anything to wait out, so it
+		// is reported as the longest TimeSpan and the caller's upper bound turns it away as too long.
+		if (totalSeconds >= TimeSpan.MaxValue.TotalSeconds)
+		{
+			duration = TimeSpan.MaxValue;
+			return true;
+		}
+
+		TimeSpan converted = TimeSpan.FromSeconds(totalSeconds);
+
+		// The check above ran on the double: a value under one tick passes it and then rounds to zero, which
+		// would start the tool and stop it in the same moment.
+		if (converted <= TimeSpan.Zero)
 		{
 			return false;
 		}
 
-		duration = TimeSpan.FromSeconds(totalSeconds);
+		duration = converted;
 		return true;
 	}