"...but don't see it"
datedif is an available function in all excel versions, but is only documented by 2000 & above. For reference, here's the help file:
<pre>
Calculates the number of days, months, or years between two dates.
This function is provided for compatibility with Lotus 1-2-3.
Syntax
DATEDIF(start_date,end_date,unit)
Start_date is a date that represents the first, or starting, date of the period.
Dates may be entered as text strings within quotation marks (for example, "2001/1/30"),
as serial numbers (for example, 36921, which represents January 30, 2001,
if you're using the 1900 date system), or as the results of other formulas or functions
(for example, DATEVALUE("2001/1/30")). For more information about date serial numbers, see NOW.
End_date is a date that represents the last, or ending, date of the period.
Unit is the type of information you want returned.
Unit Returns
"Y" The number of complete years in the period.
"M" The number of complete months in the period.
"D" The number of days in the period.
"MD" The difference between the days in start_date and end_date.
The months and years of the dates are ignored.
"YM" The difference between the months in start_date and end_date.
The days and years of the dates are ignored.
"YD" The difference between the days of start_date and end_date.
The years of the dates are ignored.
Remarks
Microsoft Excel stores dates as sequential serial numbers so that it can perform calculations on them.
Excel stores January 1, 1900, as serial number 1 if your workbook uses the 1900 date system.
If your workbook uses the 1904 date system, Excel stores January 1, 1904, as serial number 0
(January 2, 1904, is serial number 1). For example, in the 1900 date system, Excel stores January 1,
1998, as serial number 35796 because it is 35,795 days after January 1, 1900.
Learn more about how Microsoft Excel stores dates and times.
Excel for Windows and Excel for the Macintosh use different date systems as their default.
For more information, see NOW.
Examples
DATEDIF("2001/1/1","2003/1/1","Y") equals 2, or two complete years in the period.
DATEDIF("2001/6/1","2002/8/15","D") equals 440, or 440 days between June 1, 2001,
and August 15, 2002.
DATEDIF("2001/6/1","2002/8/15","YD") equals 75, or 75 days between June 1 and August 15,
ignoring the years of the dates.
DATEDIF("2001/6/1","2002/8/15","MD") equals 14, or the difference between 1 and 15 —
the day of start_date and the day of end_date — ignoring the months and the years of the dates.
</pre>
