Resolve interval handling when startAt set in the past

src/croner.js

@@ -496,7 +496,19 @@ Cron.prototype._checkTrigger = function (target) {
  * @returns {CronDate | null} - Next run time
  */
 Cron.prototype._next = function (prev) {
-	const hasPreviousRun = (prev || this._states.currentRun) ? true : false;
+	let hasPreviousRun = (prev || this._states.currentRun) ? true : false;
+
+	// If no previous run, and startAt and interval is set, calculate when the last run should have been
+	if (!prev && this.options.startAt && this.options.interval) {
+		prev = this.options.startAt;
+		const now = new CronDate(undefined, this.options.timezone || this.options.utcOffset);
+		let prevTimePlusInterval = prev.getTime() + this.options.interval * 1000;
+		while (prevTimePlusInterval <= now.getTime()) {
+			prev = new CronDate(prev, this.options.timezone || this.options.utcOffset).increment(this._states.pattern, this.options, true);
+			prevTimePlusInterval = prev.getTime() + this.options.interval * 1000;
+		}
+		hasPreviousRun = true;
+	}
 
 	// Ensure previous run is a CronDate
 	prev = new CronDate(prev, this.options.timezone || this.options.utcOffset);

test/node/js/src/suites/options.cjs

@@ -260,4 +260,25 @@ module.exports = function (Cron, test) {
 		assert.equal(nextRuns.length,5);
 	});
 
+	test("Valid interval starting in the past should give correct start date", function () {
+		const now = new Date();
+
+		const yesterday = new Date(now);
+		yesterday.setDate(now.getDate() - 1);
+		yesterday.setHours(19, 31, 2);
+
+		const sixDaysFromNow = new Date(now);
+		sixDaysFromNow.setDate(now.getDate() + 6);
+		sixDaysFromNow.setHours(19, 31, 2);
+
+		const nextRun = Cron("* * * * * *", { interval: 60 * 60 * 24 * 7, startAt: yesterday.toISOString() }).nextRun();
+
+		assert.equal(nextRun.getFullYear(), sixDaysFromNow.getFullYear());
+		assert.equal(nextRun.getMonth(), sixDaysFromNow.getMonth());
+		assert.equal(nextRun.getDate(), sixDaysFromNow.getDate());
+		assert.equal(nextRun.getHours(), sixDaysFromNow.getHours());
+		assert.equal(nextRun.getMinutes(), sixDaysFromNow.getMinutes());
+		assert.equal(nextRun.getSeconds(), sixDaysFromNow.getSeconds());
+	});
+
 };