"On Boot" is ambiguous to me. Does it mean when booting starts, when booting finishes, mid boot, when timer.target is met? The documentation that I've read doesn't resolve this ambiguity.
OnBootSec= is relative to the boot time, as given by the kernel — basically, the time at which the kernel started execution. See the relevant lines in
The table of directives in
man systemd.timer hints at this in its description of
For system timer units this is very similar to OnBootSec= as the system service manager is generally started very early at boot.
If systemd distinguishes its own startup time from the boot time, that indicates that the latter is the actual boot time (or as close as can be).