Ok, I have the following:
the edit_date is stored in the exp_channel_titles table in the edit_date field by default.
This is the value 20130320100306 (20/03/2013 10:03:06)
I tested the following tags within a ZooVisitor update form
{edit_date}
output: 08-01-1970
{edit_date format="%d-%m-%Y"}
output: 639874-06-14 00:25
{exp:low_nice_date date="{edit_date}" format="%d-%m-%Y"}
output: 14-06-1974
Running EE v2.5.3 - Build Date: 20120911
kind regards, Vic