1) Option 1: Go to a layout based on transactions. Enter find mode and specify a client ID in Bill::ClientID and put a lone = operator in the date field. Perform your find. This can be scripted or done manually.
Option 2: Put a portal to transactions on your Client layout. Define a portal filter that omits all records where this date field is not empty.
2) If the user enters the date into a global date field, the scripted find from Option 1 can use Replace Field contents to update the date field with this user specified date for all records thus found.
For examples of scripted finds, see: Scripted Find Examples
Awesome, thank you