Francesco Potortì <pot@gnu.org> writes:

> (setq diary-file "~/appt-bug-diary"
>       calendar-date-style 'european
>       )

The recipe should better call the function `calendar-set-date-style'
because "setting this variable [calendar-date-style] directly does not
take effect (if the calendar package is already loaded).

But that doesn't prevent the issue - the report is valid nonetheless.
One already sees in the diary file that the problematic lines are not
fontified.

The manual describes that different time formats can be mixed, and

  3 Feb	18.00 t3 does not work (european)

is not invalid AFAIU (`diary-entry-time': "A period (.) can be used
instead of a colon (:) to separate the hour and minute parts.").

The problem is actually that the date part is rejected because the time
format is not recognized by `diary-european-date-forms'.  This seems to
fix it: