How do I calculate hours between two 'HHmm' in Javascript?

Question

Is it possible to calculate the number of minutes or hours between two 'HHmm' strings.

Javascript example:

var now = 2050,
    next = 0850,
    minutesUntilNext = *now until then code*,
    hoursUntilNext = *minutes to hours code*;

I do not have access to the actual Date objects which is why this is slightly harder. I am however using moment.js so if you have suggestions for how to use that in this situation, that would be perfect.

Answer 1

This is pretty trivial with some basic division and subtraction:

// http://stackoverflow.com/a/10075654/560648
function padDigits(number, digits) {
    return Array(Math.max(digits - String(number).length + 1, 0)).join(0) + number;
}

/**
 * Given two times in HHMM format, returns the timespan between them
 * in HH:MM format.
 * 
 * %next is assumed to come later than %now, even if its absolute value
 * is lower (so, the next day).
 */
function compareTimes(now, next) {

   // Perform hours validation before we potentially add 24 hours (see below)
   if (now >= 2400 || next >= 2400)
      throw "Hours out of bounds";

   // If next is "earlier" than now, it's supposed to be tomorrow;
   // adding 24 hours handles that immediately
   if (next < now) next += 2400;

   // Split inputs into hours and minutes components
   now  = [parseInt(now  / 100, 10), now  % 100];
   next = [parseInt(next / 100, 10), next % 100];

   // Finally, validate the minutes
   if (now[1] >= 60 || next[1] >= 60)
      throw "Minutes out of bounds";

   // Perform the comparisons
   var minutesUntilNext = next[1] - now[1];
   var hoursUntilNext   = next[0] - now[0];

   // And return the result
   return padDigits(hoursUntilNext, 2) + ':' + padDigits(minutesUntilNext, 2);
}

console.log(doThisThing(2050, 0850));  // 12:00
console.log(doThisThing(2300, 0145));  // 02:45
//console.log(doThisThing(2500, 0000));  // error
//console.log(doThisThing(2460, 0000));  // error

Answer 2

It's just a math problem...

It could look like this:

 var now = 2230; var then = 529; function calculateMinutesDifference(now, then){ var nowMinutes = getMinutes(now); var thenMinutes = getMinutes(then); return thenMinutes >= nowMinutes ? thenMinutes - nowMinutes : (thenMinutes+60) - nowMinutes; } function calculateHoursDifference(now, then){ var nowHours = getHours(now); var thenHours = getHours(then); return then >= now ? thenHours - nowHours : (thenHours+(calculateMinutesDifference(now,then) == 0 ? 24 : 23)) - nowHours; } function getMinutes(time){ return time % 100; } function getHours(time){ return (time - getMinutes(time))/100; } alert("minutes difference: " + calculateMinutesDifference(now, then)); alert("hours difference: " + calculateHoursDifference(now, then)); 

Remember not to use leading 0 when you have hour < 10, becouse then javascript will think it's not decimal number.