The beauty of ints is you just concatenate them together, and get a bigger int.

It's only fifty days if you don't check for an overflow. If you expect the timer to have to go longer than 50 days, you can make a quick check to see if the new time is greater than the old time. If it isn't, add one to another int. This new int holds the number of times you overflowed. Multiply by fifty, and you got the whole length of days. For a regular 32 bit int, this method gives you millisecond precision out to... 18,446,744,073,709,551,615ms, which works out to 584,542,046 years.

Half a billion years should be enough for most purposes. Add another counter in case you overflow this one, and it should provide room for longer than the age of the universe.