[Konstantinos]

Use gettime without arguments/parameters when a player logins and store it in a variable. When the player disconnects, use gettime again (without arguments) and subtract (-) from the stored variable. The result will be how many seconds you were in the server so you just add the previous online time and you get the total.

About the convertion: https://sampforum.blast.hk/showthread.php?tid=309778