That is incorrect. The due date depends on a variety of factors, amount particularly, but many plans are on weekly, monthly, or quarterly deposit timing.
If the tax is not required to be remitted right away, and it is small enough to be sent in with the Form 945, it is subject to the form filing due date, typically January 31 after the year ends.
The 20% withholding is 945 tax type, so if you look for information on that, you should be able to get additional information.
Note: When the Form 945 is due is not the same as when the actual $$ must be sent in. The $$ typically have to be sent in sooner.