How do you see it possible that logged times can be consistent, when there is no agreed reference?LoTW serves its purpose, other online QSL/matching services serve their purposes, and contest logging serves its purpose, and they don't seem to have a lot in common, except that they require that the log reflects when the QSO took place.
In the times when it was mandatory to log all activity from a station, it was important that logged on-off times were precise. The same applies to contesting, where precision is an important purpose and goal of the entire activity, the QSO are very short, the established practice is to log QSOs at the off-time (and that's what contest software logs), where more than 5 minutes time difference is too much.
Many seem to hold LoTW as some sort of standard, although, with all due respect (I am a keen user, uploading after each session of QSOs), it is basically a QSO matching system for a few US awards to speed up the QSL process and simplify the award verification process. Nevertheless, LoTW demanding the start/on-time and allowing 30 minutes time difference may well be some sort of guideline for general logging, but it is useless for contesting. So, so much for consistency. If a QSL is 29 minutes off my log time, I would know that at least one of the QSO parties does sloppy logging. I would be really disappointed and surprised if it was me. I log on and off times as precisely as I possibly can, to the second. My logging practice was learnt when precise logging was mandatory and important, and then made even stricter in the Navy. This practice seems to create no problems, so I will stick to it and recommend it. And it is habitual and very easy to follow, even more so with all the tools we now have to help us log properly. All decent general logging software logs time on and time off, many (most?) can log time on automatically when you enter the call sign, and there should be no reason not to use it.