Complete the form as soon as possible, but no later than March 23th.

Complete the form as soon as possible, but not later than March 23rd.

Which sentence is correct?

Both are correct.

Both words -- "no" and "not" -- are adverbs, and both are correctly used in these sentences.

HOWEVER!!!

23th or 23rd <~~pronounce these out loud and let me know what you think!

Oops...the "th" was a typo. Guess I was still sleepy :). Thanks for the help!

You're welcome!

The correct sentence is:

"Complete the form as soon as possible, but no later than March 23rd."

In this sentence, "23rd" is the correct way to indicate the specific date of March 23rd. The suffix "rd" is added to the number "23" to indicate that it is the 23rd day of the month. The alternative sentence, which uses "23th" is incorrect because "th" should not be used with the number "23." Instead, the correct suffix for the number "23" is "rd".