In the MegaTraveller book "Referee's Companion" from GDW, ther is a section called "Keeping Time" on pp 42-45. It states that Cleon I established the calendar that was based on the Terran one.
"...The day was made exactly 24 hours, and the year was made exactly 365 days."
It doesn't mention a "galactic time" specifically, but it would stand to reason that there must be such, as Aramis stated.