From http://man7.org/linux/man-pages/man5/proc.5.html:
(22) starttime %llu
The time the process started after system boot. In
kernels before Linux 2.6, this value was expressed
in jiffies. Since Linux 2.6, the value is expressed
in clock ticks (divide by sysconf(_SC_CLK_TCK)).
stime, // stat kernel-mode CPU time accumulated by process
cutime, // stat cumulative utime of process and reaped children
cstime, // stat cumulative stime of process and reaped children
- start_time, // stat start time of process -- seconds since 1-1-70
+ start_time, // stat start time of process -- seconds since system boot
blkio_tics, // stat time spent waiting for block IO
gtime, // stat guest time of the task in jiffies
cgtime; // stat guest time of the task children in jiffies