[Issue 24704] New: DateTime.fromISOExtString Does Not Support ISO8601 Time Unit Fractions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24704

          Issue ID: 24704
           Summary: DateTime.fromISOExtString Does Not Support ISO8601
                    Time Unit Fractions
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: madric at gmail.com

## Summary

When parsing ISO8601 date-times using DateTime.fromISOExtString, the fraction
component of the last time unit causes an exception to be thrown.

## Context

The method
https://dlang.org/phobos/std_datetime_date.html#.DateTime.fromISOExtString is
built to support the ISO8601 standard:
https://en.wikipedia.org/wiki/ISO_8601#Times

According to this standard:

> A decimal fraction may be added to the lowest order time element present in any of these representations. A decimal mark, either a comma or a dot on the baseline, is used as a separator between the time element and its fraction. (Following ISO 80000-1 according to ISO 8601:1-2019,[27] it does not stipulate a preference except within International Standards, but with a preference for a comma according to ISO 8601:2004.[28]) For example, to denote "14 hours, 30 and one half minutes", do not include a seconds figure; represent it as "14:30,5", "T1430,5", "14:30.5", or "T1430.5".
>
> There is no limit on the number of decimal places for the decimal fraction. However, the number of decimal places needs to be agreed to by the communicating parties. For example, in Microsoft SQL Server, the precision of a decimal fraction is 3 for a DATETIME, i.e., "yyyy-mm-ddThh:mm:ss[.mmm]".[29]

## Steps To Reproduce the Problem

Create the following program:
```
void main()
{
        import std.datetime;
        auto b = DateTime.fromISOExtString("2024-08-15T08:13:23.000");
}
```

## Expected Results

The date-time should be parsed, with the fraction being interpreted as a
fraction of the seconds unit.

## Actual Results

An exception is thrown of the form:
```
core.time.TimeException@/dlang/dmd/linux/bin64/../../src/phobos/std/datetime/date.d(9334):
Invalid ISO Extended String: 08:13:23.000
----------------
/dlang/dmd/linux/bin64/../../src/phobos/std/datetime/date.d:9334 pure @safe
std.datetime.date.TimeOfDay
std.datetime.date.TimeOfDay.fromISOExtString!(immutable(char)[]).fromISOExtString(scope
const(immutable(char)[])) [0x5626aa88fea9]
/dlang/dmd/linux/bin64/../../src/phobos/std/datetime/date.d:3271 pure @safe
std.datetime.date.DateTime
std.datetime.date.DateTime.fromISOExtString!(immutable(char)[]).fromISOExtString(scope
const(immutable(char)[])) [0x5626aa87f85e]
./onlineapp.d:7 _Dmain [0x5626aa87ebc7]
```

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