Does anyone know what type of code this is as I am trying to decode and am stuck?

[Reddevil1976]

I am still solving cipher codes and I am stuck again :((. The code is:

Iwqgibmtz kw djtm vdrf gpw pbdp mgvz upbp wof npwu opcsu la uwpm wv dzfcwl jb uq fybkl vkl tmwgq wmct flmgmsgqa fvdtbwuqppv btme (XRL = 18 QRPRZORE 1937)

I have worked out that the bit in brackets translates as follows:

KEY = 18 DECEMBER 1937

I am not sure what this key means and I am more than a little lost.

Any help would most appreciated. Many thanks in advance.

[vinginge]

It's date shift code interpreted as 181237.
Normally, write out the message to be converted into code. Underneath the 1st letter put 1, below the 2nd put 8, below the 3rd put 1, then 2 under the next , then 3, then 7 and just keep on repeating using this sequence of 6 numbers. To convert the message to code select the 1st letter and shift it the number of letters along the alphabet represented by the number below the letter, repeat with succesive letters and numbers.
To reverse, take the coded message, write 181237 repeatedly underneath the message and shift the letters backwards along the alphabet the relevant number.
I < 1 to become H
W < 8 to become O
Q < 1 to become P
G < 2 to become E
I < 3 to become F
B < 7 to become U .....note loop round to the end of the alphabet.
Repeat for the rest of the message. The first deciphered word is HOPEFULLY.