Hi Hogen,
As per your description, I understyand that you need to compute the date where you know the no. of days before or after the date 1800/12/28. Kindly correct me if I am wrong.
You can use DateAdd().
DateAdd("d",no. of days past, #1800/12/28#)
It will return the date 1800/12/28 if the value for no. of days is 1 and date 1800/12/27 if no. of days is -1.
- Nrupal