Because decimal hours are minutes ÷ 60, not the minutes written after a point: 20 minutes is 20/60 = 0.3333 of an hour. This is the classic timesheet mistake — 1.20 decimal hours actually means 1 hour 12 minutes. If your payroll sheet expects decimal hours, always convert rather than swapping : for ..

In auto mode (the default) a value containing a colon is parsed as H:MM or H:MM:SS, anything else as decimal hours. If you want strict validation — say, catching 1.30 typed where 1:30 was meant — force the interpretation with from-clock or from-decimal and malformed input becomes an error instead of a silent reinterpretation.

Yes to both. These are durations, not times of day, so 100:30 (a hundred hours and change) is fine, and a leading - (e.g. -0:30 or -0.5) carries through to every output. The only bounds are on components: minutes and seconds must be 0–59.

Everything is canonicalised through a whole number of seconds — decimal input is rounded to the nearest second — and the reported decimal hours and total minutes are rounded to 4 decimal places. That keeps results deterministic and means the clock form, decimal form, and totals always describe exactly the same duration.