Hi PMNewbie,

If you don't have resources assigned then either the Project Calendar (set in Project Information) or a task calendar is "ruling" the schedule.

So, a couple of questions:
Where/how did you change the calendar?
Did you create a new calendar or modify the "Standard" calendar.
I'm guessing you applied a task calendar?
What version of Project are you using? You are working with Automatically scheduled tasks?

Changing the calendar should not change the duration of 120 hours -- but it definitely will change the end date.

I hope this helps.