A simple workaround is to use your own IRevisionListener. There you set the timestamp value to DateTime.UtcNow or whatever.
Assuming you're using FluentConfiguration...
var enversCfg = new FluentConfiguration();
enversCfg.SetRevisionEntity<DefaultRevisionEntity>(r => r.Id, r=> r.RevisionDate, yourRevisionListener>();
EDIT Forgot to mention... If you're using your own custom RevisionEntity (which you do above even if it is the "DefaultRevisionEntity"), you need to map this entity.