Dataflash log: TimeMS leap back

I observed the TimeMS leaped back from 993664 to 266684.
What does it mean? Is it because of reboot?

CTUN, 993633, 363, 0, 394, 0, 0.23, 0.08, 0.00, 0.00, 0, 0
IMU, 993643, 0.02417907, -0.03863649, 0.02498787, -0.3858974, 0.2380557, -9.89958
IMU, 993664, 0.005023116, -0.04608603, 0.006896142, 0.3584824, -0.2409952, -9.769167
GPS, 1, 0, 0, 0, 99.99, 48.1404201, 20.7949452, 0.22, 128.45, 31373393.92, -10988853.55, 0.07195263, 3240067889
IMU, [color=#FF0000]266684[/color], -0.009927392, -0.009696815, -0.01839858, -0.07234834, -0.04480097, -9.854527
BARO, [color=#FF0000]266703[/color], 0.7129054, 50563.03, 30.79, -0.1009069

version: APM 3.2.1