FirmwareMonotonicTimestamp and LoaderMonotonicTimestamp should be negative

[arianvp]

Note that as monotonic time begins with the kernel startup the KernelTimestampMonotonic timestamp will always be 0, and FirmwareTimestampMonotonic and LoaderTimestampMonotonic are to be read as negative values

https://www.freedesktop.org/wiki/Software/systemd/dbus/