Time Calculator

The Time Calculator works on hours/minutes/seconds and supports three operations selected by radio button: Add Times, Subtract Times, and Time Difference. Internally it never does base-60 column arithmetic directly. Instead, both inputs are flattened to a single total-seconds value with h*3600 + m*60 + s, the chosen operation runs on those integers, and the result is rebuilt with Math.floor(total/3600) for hours, Math.floor((total%3600)/60) for minutes, and total%60 for seconds. This is why overflow is normalized automatically: entering 1h 45m and adding 30m yields 2:15:00 rather than 1:75:00, and 90 seconds rolls up to 1 minute 30 seconds without any manual carry.

Inputs are parsed with parseInt, so the tool is whole-second only - there is no millisecond or decimal-second support, and any blank or non-numeric field falls back to 0 via the || 0 guard rather than erroring. The minutes and seconds boxes carry a max="59" attribute while hours is unbounded, but the math itself does not reject out-of-range values; if you type 90 minutes it is simply converted (1h 30m), so the field caps are input hints rather than hard validation. One real edge case to know: Subtract Times and Time Difference behave identically when Time 2 is larger than Time 1. The secondsToTime helper wraps the total in Math.abs before splitting it, so a negative subtraction result is displayed as its absolute value with no minus sign - subtracting 1:00:00 from 0:30:00 shows 00:30:00, exactly the same as the Difference mode.

Results are rendered in two synchronized forms: a large zero-padded HH:MM:SS readout (each field run through padStart(2, '0'), so 2 hours 5 minutes shows as 02:05:00) and a plain-language line reading the raw component values, e.g. '2 hours, 5 minutes, 0 seconds'. There is no decimal-hours or total-seconds output. A row of Quick Presets fills Time 2 only - the buttons are 1 Hour, 30 Minutes, 15 Minutes, 8 Hours, and 24 Hours - which makes it fast to add or subtract a standard shift or interval from an entered Time 1. All six input fields plus the operation choice are stored in the URL query string via useUrlStates, so a populated calculation can be shared or bookmarked through the Share button and reopened with the same values.