From the ATSC document a_65-2009.pdf (I can't get a clean link to it - just Google the name and you can see it), it's clear that the "time" is the absolute time, including the date.
From the format description of the STT,
"system_time — A 32-bit unsigned integer quantity representing the current system time as then number of GPS seconds since 00:00:00 UTC, 6 January 1980."
So it's in the meta-data that each real channel transmits. Supposed to be accurate within a second.
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