@ploum@mamot.fr are you perhaps using double quotes? With single quotes, I can't reproduce this:
$ alias t='echo it is now $(date)'
$ which t
t: aliased to echo it is now $(date)
$ t
it is now Wed Apr 15 01:41:36 PM PDT 2026
$ t
it is now Wed Apr 15 01:41:37 PM PDT 2026