Hmm...  The admin guide says it counts days elapsed since the inactivate
date.

inactivate date (due to new version or deletion)
day0    day30   day60 (delete)   day 90        day 120

ver1    ver2    ver3             ver1 exp      ver2 exp
                               vs retextra   vs retextra;
                                               ver3 exp
                                              vs retonly
                                              at 60 days

ver1 gets expired after 90 days, so day90.  ver2 would stay for 90 days,
then get expired also, so day120.  I would guess that from day61 through
day120, ver3 is compared to retextra because it's not the only remaining
version.  Then when ver2 is expired, ver3 becomes an only, the inactivate
date on ver3 is then compared to retonly (30), is found to be more than 30
days ago, and immediately expired.  So I would guess ver2 and ver3 are
expired at 90 days from ver2's inactivate date.  Unless ver3 was
deleted/inactivated more than 60 days after the ver2 inactivate date, in
which case 30 days from ver1's inactivate date would extend beyond the ver2
expiration.

Alex