Star Wars lore has more calendars and dating systems than any other fictional lore I can think of. Trying to remember them all is a pain -- it is not helped by there being conflicting dating systems even for the same time period and location (how many days in a week? Depends on which author you ask!).
I've tried to make this as accurate as possible. The years should all be correct, but days, weeks and months, I did my best with the information I found. BBY/ABY is my base. Any potential discrepencies I'll post in the references at the bottom of the page.
Current Present in SWTOR: 18 ATC
Put the year in to one field, press the "convert" button next to it, and the rest of the fields will show the year in that system. Negative numbers are BTC/BBY, positive for ATC/ABY
(Treaty of Coruscant)
(Battle of Yavin)
(Galactic Standard Calendar)
(Tho Yor Arrival)
0 Coruscant Standard Calendar (Founding of Old Republic) - starts in 25,043 BBY, which is ten years after the founding of the old Republic in 25,053 (this ten year difference is a compromise in Timeline Gold to account for a decade's difference in the dating in The Journal of Master Gnost-Dural. I chose to use this compromise date in the calculator -- if you want "years since the founding" just add 10 to the year listed). [1,6 (SWTF51V1 pf 56, entry 25,043 BBY)]
Dates from the previous "official" 10-month calendar were simply converted directly to the 12 month calendar after 2009 and we simply can say nothing noteworthy ever happened in history during month 11 and 12. That makes converting a lot easier. [1, 6]
LY is introduced in Rebels. 3258 LY = ascension of Emporer Palpatine in 19 BBY [7, 8]
The new canon time measurement was tweeted by Leland Chee on Aug 3, 2014.