A photon's journey takes no proper time
Along a light ray the proper time is zero. There is no inertial frame in which a photon sits still and wears a clock.
Proper time is the time a clock measures along its own path. For any slower-than-light path it is positive. For a lightlike path the interval is zero: emission and absorption are separated by zero proper time, however many years a distant observer counts. That is a statement about the geometry, and it is exact.
What physics refuses is the next sentence, 'from the photon's point of view.' A point of view is an inertial frame, and there is no inertial frame at the speed of light. The photon does not experience a journey of zero duration, because the photon does not have experiences or a rest frame. Observers do. Between a hot star and a retina, every possible clock that actually exists records a positive time. The null interval is what you get only in the limit you are not allowed to occupy.
Caveats
- Cosmological redshift changes a photon's wavelength between emission and absorption. That is a fact about the expansion and the two ends, not about a clock riding on the light.
- Grade is strong rather than established only because the popular wording smuggles in a rest frame. The zero interval itself is standard relativity.
Sources
- 1Einstein A.. Zur Elektrodynamik bewegter Körper. Annalen der Physik 17, 1905.