Algorithm needed to calculate difference between two times

Question

Hi,

I have an hour selection drop down 0-23 and minutes selection drop down 0-59 for Start time and End time respectively (so four controls).

I'm looking for an algorithm to calculate time difference using these four values.

Since they're not stored in fancy date/time selection controls, I don't think I can use any standard date/time manipulation functions.

How do I calculate the difference between the two times?

Answer 1

This pseudo-code gives you the algorithm to work out the difference in minutes.

It assumes that, if the start time is after the end time, the start time was actually on the previous day.

startx = starthour * 60 + startminute
endx = endhour * 60 + endminute
duration = endx - startx
if duration < 0:
    duration = duration + 1440

The startx and endx values are the number of minutes since midnight.

This is basically doing:

• Get number of minutes from start of day for start time.
• Get number of minutes from start of day for end time.
• Subtract the former from the latter.
• If result is negative, add number of minutes in a day.

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Don't be so sure though that you can't use date/time manipulation functions. You may find that you could easily construct a date/time and calculate differences with something like:

DateTime startx = new DateTime (1,1,2010,starthour,startminute,0);
DateTime endx   = new DateTime (1,1,2010,endhour  ,endminute  ,0);
Integer duration = DateTime.DiffSecs (endx,startx) / 60;
if (duration < 0)
    duration = duration + 1440;

although it's probably not needed for your simple scenario. I'd stick with the pseudo-code I gave above unless you find yourself doing some trickier date/time manipulation.

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If you then want to turn the duration (in minutes) into hours and minutes:

durHours = int(duration / 60)
durMinutes = duration % 60 // could also use duration - (durHours * 60)

Answer 2

First you need to check to see if the end time is greater than or equal to the start time to prevent any problems. To do this you first check to see if the End_Time_Hour is greater than Start_Time_Hour. If they're equal you would instead check to see if End_Time_Min is greater than or equal to Start_Time_Min. Next you would subtract Start_Time_Hour from End_Time_Hour. Then you would subtract Start_Time_Min from End_Time_Min. If the difference of the minutes is less than 0 you would decrement the hour difference by one and add the minute difference to 60 (or 59, test that). Concat these two together and you should be all set.

Answer 3

$start_time_hr = 5;

$start_time_mi = 50;

$end_time_hr = 8;

$end_time_mi = 30;

$diff = (($end_time_hr*60)+$end_time_mi) - (($start_time_hr*60)+$start_time_mi);

$diff_hr = (int)($diff / 60);

$diff_mi = (int)($diff) - ($diff_hr*60);

echo $diff_hr . ':' . $diff_mi;