I have a spreadsheet that the user inputs 4 dates (into 4 different cells)
where those dates are used in an SQL query against a database. Is there a way
to perform edit checks once the dates are input so as not to have the SQL
query execute until the dates are known to be valid ?

Base Period Start Date
Base Period End Date

Current Period Start Date
Current Period End Date

Obviously the start dates need to be before the respective end dates, but
also the Base Period needs to be prior to the Current Period. Also, no dates
can be blank, and none can be greater than yesterday.

Is there a way to execute a macro when each cell where these numbers are
input loses focus ... or am I better off with performing the total edit
checks once all 4 cells are filled in ?