How to calculate duration in days

I have recently created 2 new custom fields, that is, a date field (date1) and duration (duration1). I just want to calculate the difference in days between 'start' and dat1 in day. When I am trying to create a formula for duration1 of: start - date1 then I am getting an odd answer. Is there anyone who can give me some kind of a formula to calculate duration between 2 dates? Thanks.

Incase you want the difference in calendar days, then you can simply use the DateDiff function and if you want the difference in Working Days, then you can use ProjDateDiff function. Hope this helps.

Thank you, I tried to use the ProjDateDiff function but if I have got a different calendar than the project calendar then how can I make the distinction in the formula?

You just need to put the name of the calendar between double quotes, like for example:
ProjDateDiff([Start],[Finish],"Six Days")

Hope this helps.