It's slightly more complicated than that. For whatever reason, most of my first-run programs have a "previously-shown" date that's about a day and 3 hours earlier than the actual run time (time zone issue?). Not even the 1 day 3hr difference is a constant, however... sometimes it's 1 day 4hrs earlier, and a few programs are a full 1 day, 23hrs earlier! I'm on Pacific Standard Time, btw.
Also, sometimes the "previously-shown" date is *in* the previously-shown tag, like:
<previously-shown start="20080207000000" />)
Other times the previously-shown tag is reduced to "<previously-shown />" and there's a <date></date> tag with the earlier runtime in it, minus the 6 trailing zeros. So my script checks for a full date in either place ("full date" because for movies the <date> tag often just has the release year of the movie in it), and removes any existing previously-shown tag if the broadcast date is 1 day, 23hrs after the "previously shown" date or closer.
Hope that was clear... and hopefully my code's legible enough to explain better!