[FIX] stock,event,pos: fix "today" filter with regard to timezones

The previous filters didn't take timezones into account, and
returned stringified naive datetime values in local browser
time. Those would then be interpreted by the server-side as
UTC date, and depending on the current timezone offset vs UTC,
yield partially incorrect results.

By returning directly a datetime.datetime object instead of
a stringified version (see previous commit 76881fb2),
we can get the expected result regarless of the timezone.

Fixes #2972
Closes #6229

opw-621282