How to diff two datetimes

Today I've been looking at how to calculate the number of seconds/minutes/hours between two datetimes.

As this is both for me, and for other humans to read, I wanted to provide both the raw number, and a Xhours Yminutes view, too.

I couldn't find an online calculator that would allow me to use an ISO8601 datetime, so spent ~10 minutes doing it myself.

I started with Ruby, but it failed me being able to work this out easily, due to this gotcha, so then I found this Javascript example, which I've modified:

var before = new Date(process.argv[2])
var after = new Date(process.argv[3])

if (process.argv[2] >= process.argv[3]) {
	// swap
	const tmp = before
	before = after
	after = tmp
}

// https://stackoverflow.com/questions/18023857/compare-2-iso-8601-timestamps-and-output-seconds-minutes-difference
const diffMs = after - before
const minutes = diffMs / 1000 / 60

console.log(`Human-y: ${Math.floor(minutes / 60 * 1)}h ${(minutes) % 60}m`)
console.log(`Seconds: ${diffMs / 1000}`)
console.log(`Minutes: ${minutes}`)
console.log(`Hours:   ${minutes / 60 * 1.0}`)

From here, I can run:

% node between.js 2024-10-10T13:12Z 2024-10-31T19:45:00.000Z
Human-y: 510h 33m
Seconds: 1837980
Minutes: 30633
Hours:   510.55