What you have seems to be a UNIX timestamp, which denotes the number of seconds since midnight of 1st January, 1970.
You could convert that to a FileMaker Date by using
Let ( [
UNIXTimestamp = 1438176871 ; // yourValue
numberOfSecondsInADay = 86400 ;
days = Div ( UNIXTimestamp ; numberOfSecondsInADay ) ;
offset = 719162 // GetAsNumber ( Date ( 12 ; 31; 1969 ) )
GetAsDate ( offset + days )
or, using all those values implicitly:
GetAsDate ( 719162 + Div ( UNIXTimestamp ; 86400 ) )
Its works.. Thanks Erolst!