You can't get the last modified date for a dataset; it's not retained anywhere. You can get the last reference date in ISPF 3.4; press F11. The modified flag (I'm not sure if it's displayed by ISPF) may be of use, but it does not have a date associated with it.
Load modules and program objects have their link edit date and time stored in the member. Utilities like AMBLIST will display the date.
You can get the last modified date for PDS members with SPF statistics in the member list. Members without SPF statistics and are not load modules or program objects do not retain this information in any standard format.