You don't say whether it is just not an accurate time-keeper or whether it loses time completely e.g. when you turn it off.
In my (rather long) experience computers, which is what a phone is, are actually not terribly accurate as clocks. But if you set it to take the time from the network (in the date and time settings) that should improve it.
If the problem is loosing time completely if you remove the battery it's probably just that the backup battery on the motherboard has run out - that happens after a year or two.