try 1jan1970 without any quotation marks.
format( 1jan1970+1445432452, "mdy" )
"10/21/2015"
(That's today, for future reference. I got Epoch Converter - Unix Timestamp Converter to give me the Unix time for today: 1445432452 )
adding 1jan1970 works because the Unix time for 1jan1970 is zero. And because JMP and Unix are both counting in seconds.
Be careful, I've seen some discussion about when the Unix epoch really began: Unix time - Wikipedia, the free encyclopedia says
The first edition Unix Programmer's Manual dated 3 November 1971 defines the Unix time as "the time since 00:00:00, 1 January 1971, measured in sixtieths of a second".[5]
The User Manual also commented that "the chronologically-minded user will note that 232 sixtieths of a second is only about 2.5 years". Because of this limited range, the epoch was redefined more than once,[citation needed] before the rate was changed to 1 Hz and the epoch was set to its present value of 1 January 1970 00:00:00 UTC. This yielded a range of about 136 years, though with more than half the range in the past (see discussion of signedness above).
Craige