I think I've identified what I need to change to get what I want. If I hard code $ctime(January 1 1970 00:00:00) as the ping time, it returns [Blake PING reply]: 310292hrs 33mins 32secs like it's the person's lag time, so I just need a way to make a variable that equals yesterday's date so I can basically write (and excuse if I get the identifier names wrong):

ctcp *:ping:*: { ctcpreply $nick ping $ctime($month %olddate $year $time) | haltdef }

Thanks.