Question

I’m having trouble calculating a time (hours:minutes) difference between three time values presented as HH:mm strings.

I’m calculating time worked in a day from time in, lunch time and time out.

e.g.
var time_worked = time_out – lunch – time_in
or 07:00 = 17:00 – 01:00 – 09:00

I’m familiar with the php dates but the javascript date object is scary!

Code:

var time_in = $(".time-in").val(); // 09:00
var time_out = $(".time-out").val(); // 17:00
var lunch = $(".lunch").val(); // 01:00

var time = time_out - lunch - time_in; // should be 07:00 or 7  

Answer 1

I think you’re misconceptualizing your problem domain.

Specifically, if you’re trying to find timeAtWork, and everything is represented as a time object (e.g. in seconds since epoch), there are FOUR important times here you need to know (the three you listed [startWork, endWork and startLunch] and the one you missed, [endLunch]). In that case, you can calculate your desired value by the following subtractions, assuming that whatever language you’re working in supports adding/subtracting dates (for JavaScript, look at the excellent date.js library)

timeAtWork = (endWork-endLunch) + (startLunch-startWork)

or equivelantly

timeAtWork = (endWork - startWork) - (endLunch - startLunch)

However, it sounds to me like you have two ‘time’ variables [startWork and endWork] and one ‘timespan’ variable [lunchDuration]. In this case, lunchDuration is something like 3600 (one hour), but does not represent a true date/time combination.

In that case, lunchDuration === (endLunch - startLunch), so you can calculate timeAtWork as follows:

timeAtWork = (endWork - startWork) - lunchDuration

Hope this helps!