xul-ext-lightning 1:52.9.1+build3-0ubuntu0.16.04.1+8.0trisquel1 (Lightning 5.4.9.1): Wrong offsets for America/Sao_Paulo

When the calendar is set to use "America/Sao_Paulo" as nearest timezone, events with different timezones get a time in America/Sao_Paulo different from what GNU date --date='TZ="[Events's timezone]"' shows. Since this involves timezones, the bug depends on the event's date to be apparent.

Since the beginning of 2019, Brazil no longer differs between daylight saving or standard time. This means that now every year follows UTC-03:00. However, the problematic package doesn't know about that, as it seems to be using a timezone database separated from what GNU `date' uses.

Steps to reproduce

1. Select "America/Sao_Paulo" as the default calendar's timezone name.
2. Make an event based on other timezone name, like "America/New_York", that should happen ideally between 2019-12-01 and 2020-01-31.

Notice that the time displayed for that event is shifted to +1h, contrary to what GNU `date' shows for that same time (assuming GNU `date' is set to use "America/Sao_Paulo" as the computer's timezone name and also uses the example "America/New_Your" as the timezone name for the imput date).