Fumigation

Before detonating the bomb you could construct a rudimentary dial in a nearby park to gauge time's passage by the angle of the sun.

When police ask about your intent say "klaatu barada nikto' and inform them what you've set up in the apartment and that she's not there.

This method is guaranteed to absorb in excess of two hours.

Or you could just check the time.

I knew we could count on you.