Creating historical invoices

[John Dablin]

Encouraged by me (a long time GnuCash user) my wife is converting 
her accounts to GnuCash. She's trying to enter invoices going back 
to the start of her financial year (1st April), but when she 
creates an invoice it won't let her specify a creation date earlier 
than the 1st of the current month. Is this a feature, or is there 
something we can tweak to allow this?

Paradoxically, she can create the invoice dated 1st August, but 
GnuCash then allows it to be paid on 6th May!

Thanks in anticipation
John Dablin