Rich wrote
As a practical matter, for Part 91 operations, I've never heard of
anybody using anything but tach time.

Absence of evidence is not evidence of absence.

My Twin Comanche does not have a recording tach, because it has a
single instrument (dual needle) for both engines. Hobbs time would
have meant inspections at unnecessarily frequent intervals. My
solution was to interface an hourmeter to the squat switch using an
isolation relay. That gives me the real time in service, defined as:

In Part I, Definitions,
"Time in Service, with respect to maintenance time records, means the
time from the moment an aircraft leaves the surface of the earth until
it touches it at the next point of landing."

The truly hilarious part of this - in order to get the installation
approved, I generated so much paperwork that the weight of the paper
exceeded the weight of the installed components.

Michael