Overview

The major calendrical systems used in the Galaxy today are: (In order of origin)

Juraiian Year
The Juraiian Calendar dates from the settlement of the Jurai on their homeworld, ending a period of homeless wandering. This corresponds roughly to the year 20164 BC.  It should be noted that the Juraiian calendar did not actually correspond at first to the orbital period of their world, which seems to have somehow adjusted itself to match the calendar.  The reasons for this have never been successfully explained.

The Juraiian year consists of 18 20 day months, with four five day weeks.  The fifth day is normally a day of rest, and often a holy day.  The two solstices and equinoxes are holy days outside the normal calendar of months, as is New Year's day, the first day of the week.

The Eighteen months are:
Arch
Chill
Thaw
Blossoms
Breezes
Rains
LowSummer
MidSummer
HighSummer
SummersEnd
Gathering
Harvest
Rest
Bonfires
Frost
Freeze
Ice
Ember

BC/AD:
The old Christian calendar.  Intended to date from the birth of Christ, but usually considered slightly inaccurate in its starting point.

Serenity Year

The Serenity Year calendar dates from AD 2992, the year of the awakening of the Senshi from suspended animation.  It has gone into decline since 1000 SY, the end of the so-called Crystal Millenium.  It is chiefly used by the Cult of Serenity and on some worlds that once belonged to the Solar Kingdom.

Federal Year

The Federal Year Calendar dates from SY 1000, with the beginning of the Refugee War.  It is now the standard galactic calendar.

Conversions:
 

From	To	Change
Jurai Year	BC	20165 - JY = BC
	AD	JY - 20164 = AD
	SY	JY - 23155 = SY
	FY	JY - 24154 = FY
BC	JY	20165 - BC = JY
AD	JY	JY = 20164 + AD
	SY	SY = AD - 2991
	FY	FY = AD - 3991
Serenity Year	JY	JY = SY + 23155
	AD	AD = SY + 2991
	FY	FY = SY - 999
Federal Year	JY	JY = FY + 24154
	AD	AD = FY + 3991
	SY	SY = FY + 999