/usr/share/sensor_msgs/msg/TimeReference.msg is in libsensor-msgs-dev 1.12.3-5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | # Measurement from an external time source not actively synchronized with the system clock.
Header header # stamp is system time for which measurement was valid
# frame_id is not used
time time_ref # corresponding time from this external source
string source # (optional) name of time source
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