Calculate Day, Months & Years Between Dates

I have two columns with dates (and times) in that I am trying to define how many days, hrs and mins have elapsed i.e. A1 has 12/12/06 21:00, B1 has 17/12/06 21:00. C1 has B1-A1 and is custom formatted to show as dd"days" hh"hrs" mm"mins". In this case it will therefore show as 5days 0hrs 0mins. Which is correct.

However, if more than 1month has elapsed then the format m"m" d"days" h"hrs" m"mins" does not work. For example 17/03/06 03:00 to 20/12/06 07:00 shows as 10m 4days 4hrs 00min, which it clearly isn't.

I know the reason it does this is because it calculates the difference between the two times and adds that to it's 0 value, which in my format is 01/01/1900 00:00. therefore when it adds 277days (the answer) it becomes 04/10/1900 04:00, so my formatting is just calling the month value ('10') and the day value ('4').

I understand the reason it does this, 277 days on from 01/01/1900 is indeed Oct 4th, but 277 days on from 17/03/06 is not 10months and 4 days as there are different length months in between. It also seems to add a month on, possibly because the format for 'months' is between 1 & 12 and therefore cannot begin at 0?

Does anyone know if it's possible to force excel to work out the correct number of months and days have elapsed between two dates and not apply it to 01/01/1900? Or any other possible solution, maybe with a different custom format?