Update last modified date in a form/db

[leehbi]

I have a form connected to a SQL source table with several fields.   I have a field for DateLastModified in this table.

I need to update this field in the db when user edits the form.   On the form I have a button with SubmitForm(ProductForm)

I have the DateLastModified field on my form as an edit record with a date, hour, minute as seperate Items.

I've set the Default for the card : Now() - hence when the user navigates to the record the field updates with the current date/time.    This is saved to the db.  However, if the user has the form open for a number of minutes the data in the database will be out of date.    I need the date to be that at the time the users saves the record.

I've tried modifying the OnSelect for my save button : 

UpdateContext({DateLastModified_DataCard5: Now()});SubmitForm(ProductForm)

But this doesn't have an effect.   Is there a way to do this?