Read Help on the date functions. The best data entry method is to use one of
the date controls like DatePicker or Calendar, which is guaranteed to return
a valid date; next best is to let the user enter the date as a string then
use CDate() or DateValue() to convert it then display the converted value
back to the user. The conversions in both directions should use the user's
own regional settings rather than assuming m/dd/yy (as you seem inclined to
do -- *most* of the world does not use that format).

To calculate another date from a given date you can use the DateAdd()
function; or if you are dealing with simple day offsets you can just
add/subtract. (And forget about integers. Internally, dates are doubles.)