Time Percentage Calculator

This does percentage arithmetic on lengths of time instead of plain numbers. Enter two durations and it returns what percentage the first is of the second, or enter a percentage and one duration and it returns that share as a clock value. A third option lengthens or shortens a duration by a percentage. Durations may be written as h:mm:ss, mm:ss, d:h:mm:ss or with unit letters such as 1h 45m, and everything is worked in whole seconds so no rounding creeps in halfway. A bare number is refused rather than guessed at, since 90 on its own could mean minutes or seconds.