os is a table that has multiple time related functions. Example uses:
local time1 = os.time()
local CurrentTimeDict = os.date("*t",time1)
print(CurrentTimeDict) -- prints a dictionary of the current time, like the year, day, and week
print(time1) -- time since jan 1st, 1970, also known as unix epoch
Also os.clock() is a timer built into your computer that is a counter of seconds and is very accurate, this could be used for measuring the time taken for code to run or just a super accurate counter.
local computertime = os.clock() -- returns computer time usually accurate to the microsecond(0.000001)
Synchronization. Its a global time variable not based on location in the world or time zone. It is useful for syncing cross server or even cross game things like timed events or shop refreshes like in Arsenal.