Actually, this exact question is answered here! Take a look at this post for extra info, or have a scroll through my calendars tag.
Hope that helps!
~ Jacen
In-universe, the BBY/ABY system was really only used by the New Republic and the subsequent Galactic Alliance. The Battle of Yavin was chosen as Year 0 in the Galactic Calendar in 25 ABY, so it was not used officially before that time. Before this time, BTC/ATC (using the Treaty of Coruscant as Year 0) was a popular system. However, in 35 BBY was the Great ReSynchronization, which set that year as Year 0 (I’m not sure if you’ve found it already, but give this post a try if you want to know more about this dating system). The Empire did not use BBY/ABY, they used ReSynchronization time, though some preferred to use the formation of the Galactic Empire (19 BBY) as their Year 0.
Hope that helps!
~ Jacen
Does anyone know how to do dates in the Star Wars calendar outside of years? Are there names for standard months? I’ve only managed to find some info on the days of the week so far.
@writebetterstarwars any answers?
I don’t normally answer questions on other people’s posts, so I hope you don’t mind @operaticspacetrash, but this one is relatively straightforward. Within the Galactic Standard Calendar there are no names for months, and they are known by their number (e.g., Month 5). You probably know that there were many different variants of the calendar, all with different numbers of months in a year, weeks in a month, and days in a week. The GSC consists of 24 hours in a day, 5 days in a week, 7 weeks in a month, and 10 months in a year. These 10 months, plus 3 festival weeks and 3 holidays make up a year with a total of 368 days. In the Old Republic, the Treaty of Coruscant was used as year 0; in the New Republic, it was the Battle of Yavin.
I’m guessing that what you’re wondering about is how to write this out and refer to specific dates, like how we would say [day]/[month]/[year]? This is covered by the Great ReSynchronization. It was a notation system established in 35 BBY which used the format [year]:[month]:[day] to write dates. 35 BBY was the reference point, so, for example, the Jedi Purge occurred in the year 16 and the Battle of Yavin in the year 35. Years before 35 BBY are written with “BrS” (before resynchronization) after the number, so Qui-Gon would have been born in 57 BrS. Adding in the months and days, you get something like the date 13:5:21 (13 years after 35 BBY, on the 21st day of Month 5), on which occurred the First Battle of Geonosis. Much like how we might say “June of 2015” you can omit the day and just write [year]:[month].
This system was used through the Imperial period (though some preferred to use the Empire’s creation as year 0) and was reorganized in 25 ABY into the BBY/ABY system that we often use. Before the Great ReSynchronization and after it was disbanded, there is no canonical system for writing dates. It is possible that the [year]:[month]:[day] format was still used, but this is not confirmable.
You may also want to take a look through my tag on calendars.
Hope that helps!
~ Jacen

SOURCE. – I’m just posting it here as an aid and taking no credit.
Galactic Standard Time Measurement
- 60 Seconds = 1 minute
- 60 minutes = 1 hour
- 24 hours = 1 day
Galactic Standard Calendar
- 5 days = 1 week
- 7 weeks = 1 month
- 35 days = 1 month
- 368 days = 1 year
- 10 months + 3 Fete (Festival) weeks + 3 holidays = 1 year
Fete/Festival Weeks
- New Year Fete Week = first five days of the new year (takes place before the first month)
- Festival of Life = five days between the sixth and seventh months. It was marked by parties, get-togethers, and gift-giving.
- Festival of Stars =five days between the ninth and tenth months of the year that celebrates interstellar space travel. It was marked by vacations to other worlds most often.
Days of the Week
- Primeday (first day of the week)
- Centaxday (second day)
- Taungsday (third day)
- Zhellday (fourth day)
- Benduday (fifth/last day of the week)
The Galactic Standard Calendar is based on the
luno-solar calendar of Coruscant (from before the official founding of the Empire). It is the ‘standard’ in both the Republic and the Empire for basic measurements. However, there are many, many variations for various species, planets, etc. This is just the ‘standard’ used; similar to measurements of weight, etc.
Basically, this is not the ‘only calendar’ but the one used most often by governments, the military, etc. to be the ‘standard’. I thought it might be useful for people writing or roleplaying in the Star Wars galaxies to have a rough idea.
NOTE: There is mention in some sources of a 12 month/368 day calendar, but it doesn’t mention length of weeks, months, etc. other than ‘30-31 days’ so basically seems to be a ‘real life calendar’. I’m not 100% sure which is considered ‘canon’ so I choose to list the one that was a bit more ‘different’ for fun.
SOURCES
http://starwars.wikia.com/wiki/Galactic_Standard_Calendar
http://starwars.wikia.com/wiki/Fete_Week
http://swtor.wikia.com/wiki/Galactic_Standard_Calendar
http://starwarsrp.net/topic/63474-resourse-galactic-standard-calendar/There’s a calendar?! Whelp my fics are all wrong in terms of time now. Thank you for the extremely useful reference!
I have a notification that @bleububbletea tagged me in this, but it won’t let me actually view it for some reason. Thank you, this looks very useful!
I don’t know if anyone else has ever been sad enough to care about such things but here goes:
Click here for a quick-and-dirty google spreadsheet that converts BBY -> Imperial Year -> Republic Year -> ATC
(link is read only but you can copy to your own google drive or download)
first sheet has a converter from BBY and ATC to Republic and Imperial calendars
second sheet is a timeline of canonical events
eta: added a converter for republic and imperial calendars to the other two, and fixed the ATC > BBY conversion
sorta related: bringing this back cause it’s a useful thing. page 2 of the workbook has a timeline with major game events in
it’s worth nothing that i, like BWA, am super lazy and assume that the Empire are using the same year rollover and length as the Republic. realistically they probably would not, but BWA are lazy and so am I.
Oh man this is awesome! I was thinking about getting around to doing this (because I do use Imperial cause i’m persnickety like that and frankly I’m sick of doing math every time I need to know what year it is), but woooooo this is great!
For anyone needing to switch between calendars!