A Canadian registered charity can in certain cases provide a receipt for a cash gift that was provided before the effective date of registration.  Here is a CRA policy commentary on the issue. 

Policy Commentary
Release Date
October 4, 1993
Reference Number
CPC – 009
Subject
Official donation receipt – Whether official donation receipts can be issued by a newly registered charity for gifts received prior to the effective date of registration
Purpose
To clarify the Directorate’s policy regarding official donation receipts issued by a newly registered charity.
Commentary
1. Where a registered charity’s objects and activities are charitable throughout the year, the charity may issue official donation receipts for all cash gifts it received in the calendar year during which it was registered.
2. A registered charity cannot issue an official donation receipt for a gift-in-kind received prior to the charity’s effective date of registration. Regulation 3501(1) requires a registered charity to enter on the official donation receipt the day on which the gift-in-kind was received. For example, an organization’s effective date of registration is March 1, 1993. The organization received a gift-in-kind on January 12, 1993. Since the gift was received prior to the organization’s effective date of registration, a receipt cannot be issued for the gift-in-kind.
References
• Income Tax Regulations, C.R.C. 1978, c. 945, ss. 3501(1).