Why will it not take a plain number?

Because 90 means ninety minutes to one person and ninety seconds to another, and a wrong guess makes the total sixty times too large. Write 90m or 1:30 and there is nothing to guess. It is the one input this refuses on principle rather than because it cannot be parsed.

What notations does it take?

The clock form, where 1:30:00 is an hour and a half and 90:00 is the same thing written as minutes and seconds. And the unit form, where 1h 30m, 90m and 1.5h are all the same. Mixing them in one list is fine.

Does the total wrap at 24 hours?

No. A total of two days reads as 48:00:00 rather than resetting. A duration is a length rather than a time of day, and wrapping it would turn a two day total into nothing at all.

Duration Calculator

A list of times, added up properly.

Runs in your browserNothing you enter is uploaded, and it keeps working with the network off.

Durations

A list of times, added up properly.

How to use it

Paste the durations. One per line, or separated by commas. Both notations can be mixed.
Read the total. As hours, minutes and seconds, and as a plain number in each unit.
Check the average. Along with the longest entry, which is usually the one worth looking at.

When you would use this

Adding times up by hand goes wrong because minutes carry at 60 and hours do not carry at all. This converts everything to seconds, adds, and converts back. Two notations are accepted because both are in normal use and neither covers the other. The clock form is what a video player or a stopwatch shows: 1:30:00 is an hour and a half, and 90:00 is the same length written as minutes and seconds. The unit form is what a timesheet holds: 1h 30m, or 90m, or 1.5h. They can be mixed in one list. A bare number is refused, and that is deliberate rather than a limitation. 90 is ninety minutes to one person and ninety seconds to another. Picking one silently is how a total comes out sixty times too large, and there is no way to spot it afterwards. The total does not wrap at 24 hours. Two days reads as 48:00:00, because a duration is a length rather than a time of day.

Questions

Why will it not take a plain number?: Because 90 means ninety minutes to one person and ninety seconds to another, and a wrong guess makes the total sixty times too large. Write 90m or 1:30 and there is nothing to guess. It is the one input this refuses on principle rather than because it cannot be parsed.
What notations does it take?: The clock form, where 1:30:00 is an hour and a half and 90:00 is the same thing written as minutes and seconds. And the unit form, where 1h 30m, 90m and 1.5h are all the same. Mixing them in one list is fine.
Does the total wrap at 24 hours?: No. A total of two days reads as 48:00:00 rather than resetting. A duration is a length rather than a time of day, and wrapping it would turn a two day total into nothing at all.