Following item is valid Finnish timestamp translation:

#. This is a strftime() format. %H = hour (0-23), %M = minute.
#: ../widgets/misc/e-dateedit.c:1436 ../widgets/misc/e-dateedit.c:1763
msgid "%H:%M"
msgstr "%H.%M"

String is shown correctly in editor, but the user can't enter new dates because the editor component checks that the format matches the common nn:mm format. If the entry is manually modified from nn.mm to nn:mm the date is accepted.

Using . as time delimiter is official Finnish standard, nobody really cares about it though and everyone uses : - it still should be possible to do the right thing as well.