Hi Paul,
Create a formula with this code:
cDate(Picture(ToText({Number_field}, '#'), 'xxxx/xx/xx'))
Go to the Report Menu > Selection Formulas > Record and use this code:
{@Formula_field} IN MonthtoDate
A better approach is to create a 'SQL Expression' to convert this number field to date and use the selection formula I suggested above.
-Abhilash