Looks pretty good! Here are a couple of pointers for you:
It's custom to declare jQuery variables with a $ prefix. This way it's easy to tell at a glance that it is a element collection. So your top variables would look like this:
var $secondsDiv = $("#seconds");
var $minsDiv = $("#mins");
Instead of:
if (timer===false)
Consider:
if (!timer)
If you're looking for legibility there are probably a few other things I'd do differently.
Grouping your variables allows you to make a few shortcuts:
var time = {
seconds: $("#seconds"),
minutes: $("#mins"),
hours: $("#hours")
}
function getValue(timeStr) {
return time[timeStr].html();
}
getValue('seconds')
And I'm not sure about the check59() implementation. I'm sure we could make that more efficient, and reduce the repetition.
But the next step for you is definitely to create a class for this and scope your variables, that way you'll be able to include multiple timers in the same page :)