Where does `perf script` timestamps come from?

The third column from perf script seems to be close but not quite the uptime, where is that timestamp coming from? Is there a way to access that timestamp other than accessing sampled events?

$ perf record cat /proc/uptime
1392597.79 16669901.66
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.002 MB perf.data (19 samples) ]
$ perf script
             cat 902536 1392640.417831:          1 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.417849:          1 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.417863:          3 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.417876:         10 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.417889:         33 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.417902:        108 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.417915:        351 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.417929:       1136 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.417942:       3657 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.417958:      11701 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.417998:      34018 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.418060:      55936 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.418117:      77496 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.418195:     110190 cycles:u:  ffffffffac000163 [unknown] ([unknown])
             cat 902536 1392640.418296:     140857 cycles:u:  ffffffffac000163 [unknown] ([unknown])
             cat 902536 1392640.418422:     166643 cycles:u:  ffffffffac000b47 [unknown] ([unknown])
             cat 902536 1392640.418589:     187277 cycles:u:  ffffffffac000163 [unknown] ([unknown])
             cat 902536 1392640.418763:     198929 cycles:u:      7fdb1734b9a8 _dl_addr+0x108 (/usr/lib/libc-2.31.so)
             cat 902536 1392640.418959:     209655 cycles:u:  ffffffffac000163 [unknown] ([unknown])