OK, I believe I've found what's happening. The database column value is e.g. '11:35'. The WebGrid column displays as '11:35'. I placed the field_mask '##:##' in so that when the user edits the field, the numeric characters are required and the colon placement is maintained.

However, when the mask is applied the database ':' is displaced by the mask ':' and the DD thinks the value has changed. I don't want to give up the mask since it promotes clean data entry, so I wrote some before and after code to determine if the user actually changed these columns.

I don't have control of the format of the database column data, so I'll let it stand as is.