Tammy , you'd have to use modified_date as you can see there is no last_mod_dt in the chg object I think product could have been consistent but modified_date will do what you want ...
Note though you need to "Create" the activity association for audit_user and modified_date cause they dont exist OOTB ...
Let me know if this helps.
C:\Users\Administrator>bop_sinfo -d chg | findstr "last"
last_mod_by SREL -> cnt.id TENANCY_UNRESTRICTED
target_start_last DATE
target_hold_last DATE
target_resolved_last DATE
target_closed_last DATE
C:\Users\Administrator>bop_sinfo -d chg | findstr "date"
call_back_date DATE
start_date DATE
open_date DATE
modified_date DATE
close_date DATE
resolve_date DATE
est_comp_date DATE
actual_comp_date DATE
sched_start_date DATE
sched_end_date DATE
actual_start_date DATE
actual_end_date DATE
service_date DATE
start_date_f LOCAL INTEGER
open_date_f LOCAL INTEGER
close_date_f LOCAL INTEGER
resolve_date_f LOCAL INTEGER
est_comp_date_f LOCAL INTEGER
actual_comp_date_f LOCAL INTEGER
sched_start_date_f LOCAL INTEGER
sched_end_date_f LOCAL INTEGER
actual_start_date_f LOCAL INTEGER
actual_end_date_f LOCAL INTEGER
close_date_prev LOCAL DATE
resolve_date_prev LOCAL DATE