Date Code - An indecipherable but communicable form of code

Firstly, can any of you decipher this message?

"Remember what we did last Thursday? That again".

If you are either the sender or recipient of the code (or someone whose been following them around and tracking their every movement all week) you might know that last Thursday they met on Platform 4 at Waterloo Station if you are not then it would be completely impenetrable to you.

But how could this be applied in the field of espionage? -

Well if a spy has had to pick up a message on the 4th of october 2013 at a certain place then you would only have to send the message 4/10/13 to them for them to automatically know "Go to the same place and do the same thing as on that date".

Associations with dates could be set up in a covert way between spies if they go out casually to locations. They could go to a particular restaraunt for example, casually as friends, make a mental note of the date and then if one of them recieves that date as a message they know to meet in that same place.

ANOTHER EXAMPLE:

Computers and Cryptography experts are used to try to decipher an enemy code spoken over telephone that seems to be made up of dates but they cannot make any sense of it.

The reason they cannot is because the sender and the reciever are spies who have spent years together forming the same mental associations with specific dates; the dates are dates when they both visited certain locations and did certain things of significance to espionage. The whole spy organization is made up of these pairs who have their own shared private associations with certain dates and any of these pairs can be used as sender/reciever for secret information.