Possible boot time of operating system uses the same mechanism as uptime of operating system to calculate the times from /proc/uptime. I’ve seen some cases where the uptime was showing negative numbers but the reason was due to the /proc/uptime maintained by the OS being askew which lead to the uptime reflecting the same skew, so no fault of Bigfix.
You can check /proc/uptime in a more human readable format by using
awk '{printf("%d:%02d:%02d:%02d\n",($1/60/60/24),($1/60/60%24),($1/60%60),($1%60))}' /proc/uptime
This will show the number of days:hours:minutes:seconds that the OS is reporting its been up for which you can then compare with either the boot time or the uptime. If there is a match, it would indicate that the discrepancy is within the OS, although I can’t advise why that would be.
Some examples.
Q: boot time of operating system
A: Thu, 01 Nov 2018 04:06:42 -0600
T: 19333
Q: uptime of operating system
A: 26 days, 01:24:48
T: 7569
Q: lines of file "/proc/uptime"
A: 2251558.08 2050510.19
T: 191
awk '{printf("%d:%02d:%02d:%02d\n",($1/60/60/24),($1/60/60%24),($1/60%60),($1%60))}' /proc/uptime
26:01:24:54
A forum posting related to the same topic - How is uptime calculated for each OS?