Generate Date for the first of the following month

[Wallbl]

I have a form that has a field for the date the record was created with Date() as a default value. I also want to set a default value for a field that represents the start date of membership. If the application comes in between the 1st-15th it starts on the first of the current month. If it comes in between the 16th and the 31st the date should be set to the 1st day of the following month.

I would appreciate help on how to write the expression to set the default value for the field.

Thanks!