When does the decade start and end? Is -yy is set correctly

If you have been around long enough and went through Y2K curfuffle, then -yy (SESSION:YEAR-OFFSET) may be a dim memory.

if your -yy is 1920 then maybe you should review what happens when someone enters 20 in a 4 (four) digit date field in 2020.  

if your -yy is 1910 you may already have bad data

if -yy is 1930, put a claendar entry to check it in 2028.

cheers