Date of the letter is meaningless. I have a letter dated today inviting me to a one day BBQ on July 4. Does that mean I have until the end of today to attend the July 4 BBQ? Of course not.
What is the start date on that letter? Nobody cares about the date of March 24 unless the letter states "beginning immediately"
And no, I'm not curious as my DD is more extensive than I post. For example, I knew that it was CRA doing the stack test and not Niagara Thermal like some people foolishly thought despite how easy it was to verify.