the first line of the poster contains the "request" (11 12 11 or 1 1 1 2 1 1) that is the equivalent of the numers of knocking you hear (4 knock - 2 knock - 2 knock - 2 knock - 3 knock - 3 knock or 4 2 2 2 3 3);

the second line of the poster contains the "suggestion/solution" of the puzzle (31 12 21 or 3 1 1 2 2 1), that is the equivalent of the answer to give (1 4 3 2 2 3);

The concept behind this puzzle is that you have to undertand that the two lines of the poster are in reality a numerical sequence (and not a date and a time);

So, if the first line (question) is 1 1 1 2 1 1 and second line (solution) is 3 1 1 2 2 1, is because the first line is composed by 3 times the number 1, 1 time the number 2 and 2 times the number 1 (3-1 1-2 2-1)

If we would like to continue this numerical series, the third line after 3 1 1 2 2 1 will be 1 3 2 1 2 2 1 1 and so on;

In conclusion, if you hear 4 2 2 2 3 3 knocking (question), the solution is 1 4 3 2 2 3.

I hope that clarifies the position, but please let me know if now it is clear!