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In article 
<45fe9b50$0$17561$5a62ac22@per-qv1-newsreader-01.iinet.net.au>,
 "Clinton M James" <fakeemail@yahoo.com> wrote:

> ive been racking my brain for one simple way to convert a time but cannot 
> wok it out.
> 
> It will be in a computer program so i have the ability to use absolute 
> values if any part of the equation has a negatvie value, if required.
> 
> ok I have a spreadsheet whereby it goes through and subtracts things, such 
> as times
> 
> So say it saw 20 weeks 15 hours 12 minutes and then saw 10 weeks 22 hours 
> and 48 minutes, it would mius each column to make 10 weeks, -7 hours and -36 
> mins.
> 
> Problem is i would like to convert this result back to a normal time.
> 
> The program knows how many hours per week, such as 38 in the example given.
> 
> I need this to apply to a variety of hours, so say i have the result but a 
> different amount of hours to convert it over.. 35 hours, 40 hours, etc.
> 
> Thing is too, the hours and mins arent always in defecit as the example is 
> either, therefoer it will need to accommodate positive and negative results 
> to convert it back to weeks horus mins over the specified hours and mins per 
> week - oh thats another thing, hours per week arent always whole numbers 
> either, it could be 37.5 hours.. I prefer to work in full weeks hours mins 
> rather than breaking it into decimals so to protect data accuracy such as 
> won't be occurring when any decimials have no set end - eg, 1/3 2/3 results 
> can intefer with the accuracy, therefore working with the 3 components is 
> preferred to changing them into 1 number (37 hours 30 mins rather than 
> 37.5).
> 
> help is appreciated

Do all arithmetic (your subtractions) in terms of exact minutes.
Converting WW,HH,MM to exact minutes is easy:
    minutes = MM + HH*60 + WW*minutes_per_week
Converting minutes back to WW,HH,MM is a little trickier:
    Start with the absolute value of minutes, remembering the sign.
    WW = floor(minutes / minutes_per_week)
    HH = floor((minutes - WW*minutes_per_week) / 60)
    MM = minutes - WW*minutes_per_week - HH*60
    Finally, if the original sign of minutes was negative, negate each 
of WW,HH,MM.


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