With all the talk about the year 2000 problem, you may not be aware that Excel also suffers from a year 1900 (Y1.9K) problem. People who use Excel to store historical information often need to work with dates earlier than January 1, 1900. The only way to create a date such as July 4, 1776, in Excel is to enter it into a cell and have the program interpret it as text. Unfortunately, you can't manipulate dates stored as text ù if you want to alter their formatting, for example, or if you need to calculate the day of the week they fell on.

To address this problem, I created an add-in (for Excel 97 or later versions) called Extended Date Functions. With this add-in installed, you'll have access to eight new worksheet functions that let you work with dates in any year from 0100 to 9999. You can download a free copy of it from www.j-walk.com, or from our cover CD. The new functions are:

XDATE(y,m,d,fmt): returns the specified date (as text) in the format specified by the fmt format string (optional).

XDATEADD(date1,day,fmt): returns the date (as text) that is day number of days after date1 in the format specified by the fmt format string (optional).

XDATEDAY(date1): returns the unique day number for a date.

XDATEDIF(date1,date2): returns the number of days between two dates.

XDATEDOW(date1): returns an integer corresponding to the day of the week.

XDATEMONTH(date1): returns the month number for a date.

XDATEYEAR(date1): returns the four-digit year for a date.

XDATEYEARDIF(date1,date2): returns the number of full years between two dates; useful for calculating ages.

Both XDATE and XDATEADD functions return a text string. You can't use Excel's date formats with this string, but you can provide a format string as an argument for the function. For example, the formula below adds five days to December 1, 1895 and displays the result as 'Dec-06-1895' (these functions use standard Excel format strings):

=XDATEADD("01/12/1895",5,"mmm-dd-yyyy")

Be careful if you plan to insert dates that occurred before 1752. Differences between the historical American, British, Gregorian, and Julian calendars can result in inaccurate computations. For details, check out www.cst.cmich.edu/users/GrahamS/Pub/Doomsday/DoomsdayIntro.html.