cipher - Which door leads to freedom?

You wake up in a room. In front of you, there is some paper:

It's the day, not the year or the month! It's very useful to abecedarians. 639407344. But never forget the time, for its another key.

Jsz amxroc tt ioatblq gstk A.

There are also 26 doors, labeled A through to Z. One door leads to freedom while the others lead to a pit of acid.

Which door should you go through?

HINT 1:

He jumped on the number! UNIX!!!

HINT 2:

The word 'abecedarian' is used to describe a person that is learning the alphabet.

HINT 3:

There are no clues as to which cipher is used, so you've got to work that out from the keys. If the time is a key, then that is presumably a number. And the used cipher is similar to a Vigenere cipher. Anything come to mind, cryptologists?

HINT 4:

The only important part of the word 'abecedarian' is the meaning. A person who is learning the alphabet.

Answer

As Cristian Marian figured out, the number 639407344 is

The time/date Fri, 06 Apr 1990 13:09:04 GMT.

The question says that the day is very useful to abecedarians, which means that

The day (Friday) is going to be our alphabet key for our cipher.

Since we have the time (which is a number) as a second key, we know we're using

A Gronsfeld cipher, which is like a Vigenere but uses a number key instead of a letter key alongside the modified alphabet.

Putting it all together,

We input the cipher text with an alphabet of FRIDAYBCEGHJKLMNOPQSTUVWXZ and a key of 130904, giving us the result "You should go through door F".

So, I think I'll head through

Door F, of course!