Ham is safe to eat for up to 3 days in the refrigerator and any longer than 3 days you should freeze the ham.
Ham will frozen will be safe to eat for a year or more as long as the ham as remained frozen past the sell by date or use by date.
The sell by date is just when the store must sell the ham but it doesn't mean that the ham is bad just because it's past the sell by date.
You can keep ham for a year or more in the freezer and only for around 3 days in the refrigerator.
The freezers freezing temperatures prevents the ham from growing bacteria and spoiling so it's good well past the sell by date.
As long as the ham smells good and has no bad taste to it the ham is safe to eat for months and even a few years past the sell by date as long as you have kept it frozen.