Applying Math/Time #2

How do I apply time in my game? I''ve read there are 2 methods: 1) virtual seconds (ie. 1 second per frame) 2) real time modeling (used mostly for 3D). Since I''m doing 2D right now, how do I set my game at a locked speed of 30FPS? Obviously this must be done using ''virtual seconds'' method so your physics will appear "normal". Also, when calculating something, such as acceleration, how do I get delta t (difference in time)? This is where I''m currently roadblocked. Thanks again guys...