in the current (CVS) version of PHP4, this function
is definend in php4/ext/calendar/calendar.c
and implemented in php4/ext/calendar/cal_unix.c

it is documented in calendar.xml

a german translation is available

a japanese translation is available

this function lacks a proto definition in latest PHP3 source

there is a proto in latest PHP4 source:

int jdtounix(int jday) Convert Julian Day to UNIX timestamp