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When the RTC time at boot is off in the future by a few days, OnCalendar=
timers will be scheduled based on the time at boot. But if the time has been
adjusted since boot, the timers will end up scheduled way in the future, which
may cause them not to fire as shortly or often as expected.
Update the logic so that the time will be adjusted based on monotonic time.
We do that by calculating the adjusted manager startup realtime from the
monotonic time stored at that time, by comparing that time with the realtime
and monotonic time of the current time.
Added a test case to validate this works as expected. The test case creates a
QEMU virtual machine with the clock 3 days in the future. Then we adjust the
clock back 3 days, and test creating a timer with an OnCalendar= for every 15
minutes. We also check the manager startup timestamp from both `systemd-analyze
dump` and from D-Bus.
Test output without the corresponding code changes that fix the issue:
Timer elapse outside of the expected 20 minute window.
next_elapsed=1594686119
now=1594426921
time_delta=259198
With the code changes in, the test passes as expected.
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