Frequently Answered Question

Here is an example SAC header:

SAC> read 1994.153.17.48.43.9891.II.WRAB.00.LHZ.D.SAC
SAC> listhdr

FILE: 1994.153.17.48.43.9891.II.WRAB.00.LHZ.D.SAC - 1
-------------------------------------------------

       NPTS = 2944
          B = 1.000000e-04
          E = 2.942998e+03
     IFTYPE = TIME SERIES FILE
      LEVEN = TRUE
      DELTA = 9.999994e-01
       IDEP = UNKNOWN
     DEPMIN = -2.952368e+06
     DEPMAX = 2.598185e+06
     DEPMEN = 1.062373e+04
    OMARKER = 1730
     KZDATE = JUN 02 (153), 1994
     KZTIME = 17:48:43.989
      KSTNM = WRAB
      CMPAZ = 0.000000e+00
     CMPINC = 0.000000e+00
       STLA = -1.993360e+01
       STLO = 1.343600e+02
       STEL = 3.660000e+02
       STDP = 1.000000e+02
       EVLA = -1.047700e+01
       EVLO = 1.128350e+02
       EVDP = 1.840000e+04
      KHOLE = 00
       DIST = 2.534725e+03
         AZ = 1.169120e+02
        BAZ = 2.912198e+02
      GCARC = 2.277521e+01
     LOVROK = TRUE
      NVHDR = 6
      SCALE = 1.243780e+10
      NORID = 0
      NEVID = 0
     LPSPOL = FALSE
     LCALDA = TRUE
     KCMPNM = LHZ
     KNETWK = II
        MAG = 7.800000e+00

The origin offset is given by the OMARKER field: 1730 seconds. This is the difference between the reference time (KZTIME) and the origin time:
KZTIME + OMARKER = 17:48:44 + 1720 seconds = 18:17:34 is the origin time.