Purchased a property with tenants in place.

[Jeremie Cortez]

I purchased a property with tenants in place. I closed in December around Christmas. I gave both tenants a welcome letter informing them of how to pay rent and of emergency contacts. Both tenants are on Month to month leases. I am going today to start a new lease with one of the tenants. Can I back date it to the 1st of January or should I just start it on the 1st of February? I am in Sacramento. I am only giving month to month leases for now. Thanks in advance for your advice!