I need to enter some vouchers for the previous month can I change the system date back to last month and cause no harm? Then can I change it back to this month?

Yes, you can change the allowed dates to the previous month and that will of course affect that month’s balance. Make sure anyone who is doing In Transits, Warehouse Receipts or Invoices know that you are doing this and should either

1. Not do those things to be safe (or)
2. Be extra careful to enter in the right dates for invoices etc.

Basically you need to be careful no one affects the previous month’s data while the vouchers are added. After the vouchers are added, you can change the dates back to this month.