“No later than” is correct and means the latest time that something can happen(a deadline). “No later then” is incorrect and should not be used in English.
Than vs Then
Than and Then are two words that sound very similar but actually have two different meanings and uses.
“Than” is a conjunction and a preposition.
We use “than” to compare things, especially with comparative adjectives.
I am taller than Susan.
He is richer than me.
“Then” is an adverb and an adjective.
We use “then” in the following situations:
1. At a certain time
The movie was at 8 pm. What were you doing then?
2. To introduce effect in cause and effect
If you drink too much, then you will have a hangover.
3. To introduce what happens next
We went to the park and then we went to the beach.
4. In Addition
You need to pay for the ticket and then you need to pay for parking as well.
“No later than” meaning and when to use
“No later than” is a formal way to introduce a deadline.
You need to hand in your paper no later than 5 pm on the 6th of May.
I will be back no later than 5 o’clock.
“No later than” is an adverb phrase which means that the phrase “no later than” functions as an adverb in a sentence.
This means that the adverb phrase modifies the verb(adds information), In the first example above ”no later than” modifies the phrasal verb “to hand in”.
I would use “no later than” in more formal situations. It is more common in professional situations like in the office or at a University.
For example, I would never say the second example above to somebody I know well although this is more common in American English.
I would say “I will be back before 5 o’clock”
No later than vs Not Later than
“Not later than” means the same thing as “No later than” and is even more formal.
You only see “not later than” in formal writing such as rules set by the government or law contract.
The invoice must be paid not later than the 6th of May.
“By no later than” meaning
“By no later than” also means the same thing as “no later than”.
The word “by” is also used to introduce a deadline and is a little bit redundant in this case.
You need to be home by midnight.
You need to be home no later than midnight.
No later than or then conclusion
The main confusion about “no later than” or “no later then” is because “than” and “then” sound similar.
It is also confusing because “then” is often used with time.
“Than” is used with comparison and in this phrase it is quite difficult to understand what we are comparing.
The comparison is the time after the deadline and the deadline but it makes things so much easier to express a deadline in the time before the deadline.
Some easier synonyms that are used in everyday English are “at the latest” or “before”. The opposite of “no later than” is “no earlier than”.
So now you understand why it is “no later than” and not “no later then”!