Three ways to measure elapsed time
"Between two times" answers the most common version of the question: given a start and an end clock reading, how much time passed. "Add/subtract" answers a different question: given one time and a duration, what time results, useful for figuring out when a task that started at 2:15 PM and runs for 3 hours 40 minutes will finish. "Two date-times" is for spans longer than a single day: enter a full start date and time and a full end date and time, and the result includes a day count along with hours and minutes.
All three modes convert clock readings into whole seconds first, then do the arithmetic in seconds before converting back, so a result of "8h 30m" and a result of "8.5 hours" always describe the identical span, just in two common formats. To take that resulting span further, into weeks, months, or years, feed the total into the time converter.
Why an earlier end time means the next day
A search for the time between two clock readings almost always means a single continuous span, not a full day minus that span. If the end time entered is earlier than or equal to the start time, this calculator assumes the missing hours belong to the next calendar day rather than returning a negative number. A shift that runs from 10:00 PM to 6:00 AM, for instance, is 8 hours: the clock passes midnight partway through, and 6:00 AM only reads as "earlier" than 10:00 PM because it belongs to the following day.
This is the convention every general-purpose elapsed-time tool uses for the same reason: on a plain 24-hour clock with no date attached, there is no way to distinguish "2 hours" from "26 hours" without picking one interpretation, and the overnight-span reading matches how the question is normally asked.
Worked example: a 9:15 AM to 5:45 PM workday
Start at 9:15 AM (33,300 seconds after midnight) and end at 5:45 PM (63,900 seconds after midnight). The difference is 30,600 seconds, which breaks down to 8 hours and 30 minutes, or 8.5 hours exactly, since 30 minutes is precisely half of 60. A workday quoted as "9 to 5" with a half-hour added on either end lands on this same 8.5-hour figure.
Payroll and time-tracking software often display both formats side by side because they serve different purposes: decimal hours multiply cleanly against an hourly rate (8.5 hours at $20/hour is $170.00 with no further conversion), while hours and minutes reads naturally off a clock.
When to use "Two date-times" instead
"Between two times" and "Add/subtract" both work on a single abstract 24-hour clock with no calendar date attached, which is enough for anything within one day or one overnight rollover. "Two date-times" is for a longer span: a project that starts at 9:00 AM on January 1 and finishes at 9:00 AM on January 3 is exactly 48 hours, 2 full days, a figure the single-clock modes cannot express since they only ever assume at most one day has passed.
The two calculations agree wherever they overlap. An overnight span entered as 10:00 PM to 6:00 AM in "Between two times," and the same span entered as two full date-times one calendar day apart, both return 8 hours, because both ultimately convert the clock readings to seconds and subtract.