[BUG] Fix how the start time for a given schedule is computed for efficiency

[WWGolay]

The t0 variable in scripts like exoplanet_transits and schedtel is the observatory's local noon, stored in UTC. The rst script calculates this in the most efficient way using timezones; other scripts should also use this way:

tz = timezonefinder.TimezoneFinder().timezone_at(lng=lon.deg, lat=lat.deg)
tz = zoneinfo.ZoneInfo(tz)
logger.debug(f"tz = {tz}")

if date is None:
    logger.debug("Using current date at observatory location")
    date = datetime.datetime.now()
else:
    date = datetime.datetime.strptime(date, "%Y-%m-%d")
date = datetime.datetime(date.year, date.month, date.day, 12, 0, 0, tzinfo=tz)

t0 = astrotime.Time(
    datetime.datetime(date.year, date.month, date.day, 12, 0, 0, tzinfo=tz),
    format="datetime",
)
logger.debug(f"t0 = {t0}")