start and end dates are considered as UTC and not as local time

When filling the form to register an experiment or instrument, we have found that selected start and end dates were being considered as local dates, not as UTC, in the client side (server validation has been working fine). That is causing some unexpected and wrong behaviour like the following case:

• If your timezone is, for example, UTC-7 and you selected 2026-04-22 as the start date when your local time is 2026-04-21 16:50h (almost 2026-04-22 in UTC), a validation error will appear because the date part of the selected start date (which will be considered as 2026-04-21 17:00h UTC) will be 2026-04-21 and then you will be expressing that you want to start the experiment at 2026-04-21 14:30UTC which is the past.

With this patch the selected start/end dates are directly considered as UTC start/end dates and, for the case above, the considered date part would be 2026-04-22 (and not the day before)

Bug: T422679