Ticks between Unix epoch and GPS epoch

Question

What is the number of one second ticks between Unix time epoch (01 Jan 1970) and GPS time epoch (06 Jan 1980)?

I have seen multiple answers from several sources on t

Answer 1

I guess I'm in the third camp :)
Let's call it like it is:
2,904,548,141,415,381,930 "periods[...]of a caesium 133 atom" measured at 0 degrees Kelvin at the geoid. (give or take a few hundred million periods depending on which TAI/SI definition you use)

Short Answer:

It depends on what time scales (and which definitions of those time scales) you're using.

315964809 in TAI seconds (1977 definition) and thus UTC seconds
315964800 in UNIX seconds
(both are equal to eachother but ONLY between your specified dates and both correspond to 2,904,548,141,415,381,930 "periods[...]")
Please note that UNIX seconds replay the same second after the completion of a UTC leap second, so that the UTC seconds, 2012-06-30 23:59:60 UTC and 2012-07-01 00:00:00 UTC, were both represented by a UNIX timestamp of 1341100800.

Detailed Answer:

Using TAI seconds
Even though they aren't really, let's assume that all TAI seconds before 1977 are still exactly equal to the 1977/1997 definition of TAI/SI seconds.
Let's also assume that by
"Unix time epoch (01 Jan 1970)" to "GPS time epoch (06 Jan 1980)"
you mean
1970-01-01 00:00:10 TAI to 1980-01-06 00:00:19 TAI
in this case there would be
( ( (365days/year * 10years) + 2 leap days + 5 days) * 86400 TAI seconds/day ) + 9 TAI seconds
= 315964809 TAI seconds

Using UNIX seconds
Even though they aren't really, let's assume that the duration of a UTC second before 1977 is still exactly equal to the 1977/1997 definition of a TAI/SI second.
Let's also assume that by
"Unix time epoch (01 Jan 1970)" to "GPS time epoch (06 Jan 1980)"
you mean
1970-01-01 00:00:00 UTC to 1980-01-06 00:00:00 UTC
and that UNIX time skips back a second after the completion of a leap second
in this case there would be
( ( (365days/year * 10years) + 2 leap days + 5 days) * 86400 seconds/day ) + 9 leap seconds - 9 unix leap second rehashes
= 315964800 UNIX seconds

Concerning "Periods[...]"
A 1977/1997 TAI/SI second is what was used to come up with 315964809 seconds of 9,192,631,770 periods each = 2,904,548,141,415,381,930 periods. An 1997 SI second is equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom at rest at a temperature of 0 K. The 1977 definition of TAI measures SI seconds at the geoid.