stat: incorrect rounding of fractional pre-epoch timestamps with precision

## Description

`stat` produces incorrect output for fractional timestamps before the Unix epoch when precision is specified.

For example:

```text
-0.876543211

%.1Y -> -0.9
%.3Y -> -0.877
```

GNU coreutils produces:

```text
%.1Y -> -0.8
%.3Y -> -0.876
```

The fractional value should be truncated toward zero when reducing precision.

Related: #14705  
