You have two options here.
In both cases, you should set the tenancy start date to reflect the actual tenancy start date.
After that, the way that rents are applied is determined by the Opening Balance Date for that property, i.e. This is the date from which you wish to manage this property using RentPro. Note that if you enter a transaction with a date prior to the Opening Balance Date, such as a rent received or a sundry fee, then it will be ignored.
Option 1 - start from now (recommended)
If you set the opening balance date to today, then it will only start applying rents and considering fees, etc. after this date.This is the quickest and easiest method.
Option 2 - start from the tenancy start date
If you set the opening balance date to be the actual tenancy start date, then you will have to enter all rents received, payments to landlords, support tickets, etc. retrospectively. If the tenancy start date is considerably long ago this could mean entering a lot of data, so you need to weigh up the time spent versus the actual benefit.