Android : Need To Calculate Time Elapsed - While Developing Game

So I've been struggling with this and I am not sure why because it should be simple, but for some reason I just can not get my head around it !!

I am developing a game and when the user saves their game, I write out the last time I checked their stats using System.currentTimeMillis(). Then when they load in the game I want to see how many total seconds have elapsed since it was last saved. Naturally I thought of getting System.currentTimeMillis() and then deducting my saved value and then converting that into seconds using this :=

long timePassed = System.currentTimeMillis() - lastCheck; int seconds = (int) ((timePassed / 1000) % 60);

But on each load the seconds keep altering, but not within the right bounds. For example on one load it comes as 59 seconds, then it showed 23 seconds etc. Obviously my algorithm is slightly wonky but I can't work out why.

Could some one please elaborate where I am going?