I created a validation rule for some cells which won't allow changes to the cells based on date.

For example on August 1 no entries are allowed to cells in columns for Jan - July.


It works when the user tries to enter new data but doesn't work with the user simply copies data from elsewhere and simply pastes into the column like June.

This, hence, isnt' as robust a validation as I would like. Any thoughts on how to make it more secure?