It’s not the date you put in the letterhead. That would be insane.
"or anywhere else" is an exceedingly broad assertion. For Canada:
> Yes, you do have to give notice of your resignation in Canada. The common law imposes a duty to provide notice of resignation on all employees.
> However, you don’t have to give two weeks’ notice of your resignation in Canada per se. Rather, you have to give a “reasonable” amount of notice of your resignation, which may be more or less than two weeks’ notice. The amount of reasonable notice an employee has to give will depend on their specific circumstances, as discussed below.
> The obligation to give reasonable notice is a general common law obligation of all employees. In Sure-Grip Fasteners Ltd. v. Allgrade Bolt & Chain Inc., [1993] 45 C.C.E.L. 276 (Ont. Gen. Div.) at pages 281-282, Justice Chapnik found: […]
* https://duttonlaw.ca/do-you-have-to-give-two-weeks-notice-in...
During the 'notice period' you still have a job, though the employer may tell you to simply stay at home (and rescind access, etc).
Clarification: I put in the body of the letter the effective date I will be leaving. Something like:
"My final day will be December 12, 2018."
The date in the letterhead has nothing to do with it.