How Long A Table Blocked

I have written a stored procedure to list out all tables in which
rows or the table itself is locked. The only information I am
not able to get is the time when the lock occurred. The way I
want is that if I run the procedure it should show all locks
on a table which are at least 5 or x seconds old. This way I can
avoid momentary locks on a table which go away after few seconds.

Which table and column of master database has that information?

Thanks.

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