Use default TZ when calculating birthdate if on_date param is not set

[xispa]

Description of the issue/feature this PR addresses

This Pull Request makes the system to use system's default Timezone when calculating the date of birth via api.get_birth_date if no value for parameter on_date is explicitly set.

As a result, this fixes the following test:
https://github.com/senaite/senaite.patient/actions/runs/10426342600/job/28879098625#step:7:226

Current behavior before PR

System does not take system's default TZ when calculating birthdate if on_date is not set

Desired behavior after PR is merged

System does take system's default TZ when calculating birthdate if on_date is not set

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.

[ramonski]

Thanks! And indeed, the TZ settings are challenging.
Maybe we should always use instead the TZ of the current logged in user in the future, which you can configure in the User Profile...

[xispa]

Thanks! And indeed, the TZ settings are challenging. Maybe we should always use instead the TZ of the current logged in user in the future, which you can configure in the User Profile...

Yes fully agree.

For the records, there is the function plone.app.event.base.default_timezone that can be used to get the user's TZ (and fallback to site's default as well). I think is worth to add a kind-of shortcut function senaite.core.api.dtime.get_user_timezone(user=None) like we have for os (senaite.core.api.dtime.get_os_timezone). Or even a get_default_timezone() that relies on get_user_timezone and fallsback to get_os_timezone if not a valid TZ