exiv2 allows the user to set invalid dates

[infinity0]

Describe the bug

exiv2 allows the user to set invalid dates, such as those in the common format YYYY-mm-dd HH:MM:SS.

In EXIF the dashes must be colons instead. exiv2 doesn't check this, allowing the user to set an invalid date without any warning.

To Reproduce

Find any JPEG file and copy it to test1.JPG

$ x=test1.JPG
$ d="2023-07-21 21:06:26"
$ exiv2 \
  -M "set Exif.Image.DateTime $d" \
  -M "set Exif.Photo.DateTimeOriginal $d" \
  -M "set Exif.Photo.DateTimeDigitized $d" \
  "$x"

Expected behavior

exiv2 should not let me set date strings that are not formatted correctly as per the EXIF specification, at least not without a clear obvious warning.

Additionally, exiv2 should print warnings about invalid dates. At the moment, this doesn't occur even with -v.

Desktop (please complete the following information):

• Exiv2 version and source: (e.g., 0.27.6 from Debian)

Additional context

• Invalid dates were only detected via a lucky observation from exiftool Inconsistency constructing composite datetime written by exiv2 vs exiftool exiftool/exiftool#213