constructor(delayMs, callback) {
		this.delayMs = delayMs
		this.callback = callback
		this.counter = 0;
		this.start = Date.now();
		this.active = true;
		// start timer
		setImmediate(() => this.delayed())
	}
Create a timer with the given delay.
Important: will run once when started, then once every delayMs.