How are flight hours counted?
I can answer part of this by experience, as a non-professional hobbyist pilot, and partly by information which is fairly easy to come by. So here goes:

Are fractions of an hour taken into account or only ever full hours? I.e. if some pilot flew 1:27 h on one flight and 1:33 h on another how does that count towards the logged flight hours?

Fractions of hours are absolutely taken into account. Typically accuracy is not to the minute, as this would be fairly pointless but typical schemes are to round to the nearest 1/10th of an hour (6 minutes) or 5 minutes. Personally, a long time ago I chose to go for 5 minute rounding and have regretted it ever since - 6 minutes is much easier to add up when dealing with hours &amp; minutes!

Given many regulations surrounding aviation seem to be global, is the way flight hours are logged uniform around the world?

Nothing in aviation is global, or very little. Although there are bodies such as ICAO (International Civil Aviation Organisation) which most member states tend to derive most of their rules from. For the purpose of logging flight time, I expect most civil pilots log on the basis of &quot;Chock to chock&quot; time. Which is to say when the aircraft moves under it's own power until it stops.

Any differences between commercial, private and military aviation?

From what I can tell, the only difference is military which seem to log in-air time. This, I'm told, is why military airmen who intend to pursue a commercial pilot career after they're discharged maintain a separate record of their flight time.