use the datediff function
Returns the number of intervals between two dates.
DateDiff(interval, date1, date2 [,firstdayofweek[, firstweekofyear]])
The DateDiff function syntax has these parts:
Arguments
interval
Required. String expression that is the interval you want to use to calculate the differences between date1 and date2. See Settings section for values.
date1, date2
Required. Date expressions. Two dates you want to use in the calculation.
firstdayofweek
Optional. Constant that specifies the day of the week. If not specified, Sunday is assumed. See Settings section for values.
firstweekofyear
Optional. Constant that specifies the first week of the year. If not specified, the first week is assumed to be the week in which January 1 occurs. See Settings section for values.
example
datediff("d","1/1/2003","1/12/2003"