Why do you need a unique ID in the Date field? What is the purpose?
There are other ways to create a unique id.. such as Get (UUID)
Perhaps have another Calculation field, based on the creation date and the Serial Number.
So you'd have 3 fields.
SerialNumber = Auto-generated Number on creation (i.e. 001)
Date = Auto-generated Timestamp on creation (i.e. 25/11/2013 16:21:43)
DateBasedID = GetAsNumber ( Day ( Date ) & Month ( Date ) & Year ( Date ) ) & SerialNumber (i.e. 25112013001)
However, like Jason, I can't think why you'd need to have this.
Often such "secret decoder ring" Id's are needed to support labeling uses outside the database (such as a label put on a shipping carton) or to support legacy systems clients are unwilling to abandon. Such uses can be supported by FileMaker , but please do not use such a value as a Primary key for linking the table to other tables in your database.
A self join relationship can be set up that matches by a date field and then Set Field can be used in a script for creating new records. The script can create a new record, a date field used in the relationship can auto-enter the creation date and then a Set Field step can assign the date based ID number:
Set Field [YourTable::Sequence ; Max ( YourTable|SameDate::Sequence ) + 1 ]