The following should do the job.
Invdate: DateSerial(Year([Jobdate]), Month([Jobdate])+1, 1)
Creates a date of 1st of the month following date entered.
Reccashbydate: DateSerial(Year([Jobdate]), Month([Jobdate])+2, 1)
Creates a date of first of month, 2 months after date entered.
Need to set...