The main point of Colonial Standard Time is so that colonists can order their days to the natural rhythm of the planet. It is also an important element in establishing a unique cultural identity for the colony. For scientific and technical matters Terran Standard Time units would still be used but on an everyday social level Colonial time would take precedence. There would also be a Colonial Calendar based on the planetary year with leap year days inserted where necessary.
While some worlds might choose to anchor their calendar on the anniversary of the colony's founding others might use a local equinox or solstice to align their calendar more comfortably with the local seasons. Local weekday/month names and week/month lengths would vary from colony to colony. Some keep traditional Terran names, others choose a numerical name system while still others select names in an arbitrary fashion, but period lengths are usually chosen to divide as evenly as possible into the calendar year. Again this is mainly an issue of
Colonial Identity and can be a very political one. For interplanetary matters the Terran Standard Date would remain in use.
Dalton “So when is your birthday again?” Spence