The Empire uses Terran units of time as its standard, and the rest of the Reach follows perforce. The standard day has 24 hours and the standard year has 365 days, with intercalary days added when necessary. However, in places where interstellar trade or the Empire are not pressing concerns, local calendars may predominate.
The Imperial calendar does not concern itself with months. The four chief holidays of Ideolatry keep a faint echo of Earth's solstices and equinoxes, and the Empire retains the seven-day work week.
An Imperial time-stamp looks like:
587:044:19:22:09.919047...
(yr:day:hr:mn:sec) The year is dated from the founding of the Empire (Ad Imperii Condite, AIC).
New Year's Day: day 1
Founding Day: day 84
Nature Day: day 91
Justice Day: day 182
Wisdom Day: day 273
Pantheolia: day 280
Terraform Day: day 298
Leap Day(s): days 365 (& 366)
Emperor's Birthday: moveable
Ascension Day: moveable
weekends: every 7 days
Easter: moveable, c day 90
St. Timon's Day: day 120
Admiralty Day: day 132
League Day: day 215
Invasion Day: day 298
Secession Day: day 313
Christmas: day 359
Sabbath: every 7 days
Cleansing Days: every 14 days
Patham's Night: every 194.7 days
Feast of All Forms: every 454.2 days
Archetypes' Day: 100 days after Feast of All Forms (lasts 1 to 3 days)
Easter, Christmas, and Sabbaths are, of course, Christian holidays.
St. Timon's Day and Cleansing Days are Timonian holidays.
The Feast of All Forms and Archetypes' Day are Pantomorphean holidays.
Admiralty, League, Invasion, and Secession Days are political commemorations, illegal or discouraged in many places, especially the Empire.
Patham's Night is a popular holiday adopted from the Patham cult.