Very simple. What it does is this:
((Wikipedia:Wikipedia Signpost/Single|issuedate=((#titleparts:((FULLPAGENAME))|3|3)))) [[Category:Wikipedia Signpost Single archives ((#invoke:string|sub|((#titleparts:((FULLPAGENAME))|3|3))|1|4))|((#invoke:string|sub|((#titleparts:((FULLPAGENAME))|3|3))|6|7))((#invoke:string|sub|((#titleparts:((FULLPAGENAME))|3|3))|9|10))]]
This looks monstrous. Here is what it does:
((#titleparts:((FULLPAGENAME))|3|3))
.The magic word "titleparts" works like this:
Wikipedia:Wikipedia Signpost/Single/1969-12-31 ^ ^ ^ 1 2 3
We can then take substrings of the third part:
1969-12-31 0000000001 1234567890 ^ ^ ^^ ^^
i.e. 1 to 4 is 1969
, 6 to 7 is 12
and 9 to 10 is 31
.
So we are categorizing it like such: [[Category:Wikipedia Signpost Single archives 1969|1231]]