extract finding time until $interval function

Author: hawkrives

modules/timer/__tests__/timer.test.js

@@ -0,0 +1,22 @@
+/* eslint-env jest */
+
+import {msUntilIntervalRepeat} from '../index'
+
+describe('msUntilIntervalRepeat', () => {
+	test.each`
+		now              | interval | expected
+		${100}           | ${10}    | ${0}
+		${1536625260000} | ${60000} | ${0}
+		${60000}         | ${60000} | ${0}
+		${35}            | ${60}    | ${25}
+		${110}           | ${100}   | ${90}
+		${1}             | ${9}     | ${8}
+		${1000}          | ${20}    | ${0}
+		${9}             | ${20}    | ${11}
+	`(
+		'returns remaining: $expected with now: $now and interval: $interval',
+		({now, interval, expected}) => {
+			expect(msUntilIntervalRepeat(now, interval)).toBe(expected)
+		},
+	)
+})

modules/timer/index.js

@@ -4,6 +4,14 @@ import * as React from 'react'
 import moment from 'moment-timezone'
 import delay from 'delay'
 
+export function msUntilIntervalRepeat(now: number, interval: number) {
+	let next = now
+	while (next % interval !== 0) {
+		next += 1
+	}
+	return next - now
+}
+
 type Props = {
 	interval: number, // ms
 	timezone?: string,