One page, deliberately
This family currently has a single page. That is not an oversight — it is a test. Hour offsets sit right next to the class of query where a search engine renders its own clock or countdown widget, and in that class the top organic result keeps a single-digit share of the clicks. Building 175 pages to find that out afterwards would be 175 pages of wasted work, so one page goes live first and the rest are built only if it earns anything.
What an hours page can and cannot say
Every other page on this site can be correct in static HTML, because its answer depends on a date and the build knows what date it built on. An hour offset depends on the time of day, which the build cannot know and which is different for every visitor. So the page does not invent one: the static content is a conversion table covering all twenty-four possible starting hours, and your browser fills in the exact time from your own clock.
The other thing an hours page has to be honest about is that a day is not always twenty-four hours long. On the two days US clocks change, adding wall-clock hours and adding elapsed hours give answers an hour apart. The time calculator shows both. Counting in whole days is unaffected — a clock change cannot move a calendar date.
How the offset is counted here
From the moment the page reads your device's clock, in wall-clock hours, in whatever zone that device is set to — not from the top of the hour, and not from the moment the page was built. Adding six hours to 21:40 gives 03:40 the next day, and the page names that day, because the part people actually get wrong about an hour offset is the date rather than the time. Minutes are carried rather than rounded, so the answer keeps the minute you started from, and the 24-hour form is printed alongside the 12-hour form on every line.
If what you have is a written 24-hour time to convert rather than an offset to count forward, the military time converter is the page for that. For the gap between two fixed times, use the time duration calculator.
Hours, days and time zones are three different questions
"Six hours from now" is a clock question and its answer changes every minute you delay asking it. "Six days from now" is a calendar question with the same answer all day. "Six o'clock, but in Berlin" is a zone question and depends on nothing you are holding. This site keeps the three in separate families on purpose, because conflating them is where wrong answers come from — most obviously on the two days a year when a wall-clock hour and an elapsed hour are not the same length. For a span in days go to days from today; for the same instant somewhere else go to time zone conversions.