Since the earliest days of radio transmitting, individuals and organizations have made an effort to record and preserve radio signals in the form of broadcasts and other over-the-air communications, especially those of historical significance. Now low-cost software-defined radios (SDRs) coupled with today's faster memory-enhanced computers allow us to record not just individual signals from one radio station at a time, but an entire broadcast band - a wide swath of frequencies - all at once. Each recording from a particular day and time can easily contain dozens, if not hundreds, of stations broadcasting and communicating simultaneously. Later, via a software-defined radio application, recordings can be tuned and listened to (decoded) as if they were live. This talk will discuss how you can build your own "radio time machine" which supports such virtual time shifts by utilizing an inexpensive ($25-$100) SDR, and also show how you can - for free - virtually "travel" through recent history on radio archivists' preexisting radio time machines.