Part 1: gnus-summary-limit-to-age

`gnus-summary-limit-to-age' (`/ t') in the summary buffer prompts for
a number with the string "Limit to articles within (in days): " or
"Limit to articles older than (in days): " (with prefix argument `C-u
/ t'). Looking at the code I noted that it accepts negative arguments
too in both cases, switching between older and younger [1]. 

WIBNI the prompt string would tell the user the possibility to enter
negative numbers? Here's a patch against gnus-sum.el: