Calculating a variable from an input field?

[EverleighPresley]

I am sure I am missing something very straightforward, so some assistance would be appreciated. In my load script I load a master calendar, Which has a range of dates corresponding to the range of dates in the transaction file, plus the number of the week in the year. The date links to a date field in the transaction file I am loading. I want the user to enter a date (any date in the range of the master calendar) which I store in a variable I called 'vInputDate'. Based on the date the user enters I want to assign a variable the corresponding value of the week variable.

[edwin]

if the user selects only one date, then only one week is possible.  you can just straight away assign the possible week selection by 

=only(week)  //assuming week is the field for the week number