This is incorrect from every single English usage discussion I have seen on this matter. "By" means "on or before", inclusive of the period in question. "Before" is not inclusive of the period in question.
Some google references:
Google
Google
Sorry for putting on the grammar nazi hat
.
In practical usage, many times this difference does not matter, so they are used interchangeably, but in some cases it does matter (for example for deadlines). An example that affects me personally is the IRS tax payment deadlines. Here's what it says on the IRS page, just as a concrete example:
"If you don't pay enough tax
by the due date of each payment period, you may be charged a penalty even if you're due a refund when you file your income tax return at the end of the year.
If you mail your estimated tax payment and the date of the U.S. postmark is
on or before the due date, the IRS will generally consider the payment to be on time. If you use IRS Direct Pay, you can make payments up to 8 p.m. Eastern time
on the due date. If you use a credit or a debit card, you can make payments up to midnight
on the due date."
Individuals
So a literal reading of what Elon said is anytime in March is fair game.