invoice chooses it's own date!

The lady that creates our invoices has complained for a couple of years now that sometimes, and we can't find a common thread as to why, but sometimes the invoices date back a month or two , then sometimes dates it in the future.

I've asked our Microsoft GP consultants and they don't know why this happens. 

We need to get to the bottom of this.

Any ideas?

Thanks,

Pam