Upvote:5
I have never seen a letter for those two countries, only for Canada-USA and Canada-Europe. With that in mind, in my experience the letter itself contains the period. For example
I consent to X accompanying Y for the period A to B visiting C and D.
Using just a single date as the start of your consent leaves things open to interpretation. I would not risk it or presume to know how officials would interpret a letter that did not include an explicit period of validity.