# Arduino millis\(\) timing breaks after days: use unsigned long and subtract

> Fix timers that fail after about 49 days or misbehave early: store millis\(\) in unsigned long and compare with subtraction.

Canonical: https://roboticsengineering.in/fix/arduino-millis-timer-overflow-use-unsigned-long/ | Published: 2026-11-03 | Updated: 2026-11-03

**Store millis() in an unsigned long and compare by subtraction.** Written like that, the timer survives the roll-over.

## What the message means

millis() counts up to a large number and then starts again from zero. A signed or smaller variable overflows sooner and breaks the comparison.

## Fix it in this order

1. Declare the variable as unsigned long.
2. Use if (millis() – last >= interval) and then set last = millis().
3. Do not write if (millis() > last + interval).
4. Do not store millis() in an int, which wraps after about 32 seconds on an Uno.
5. Use micros() for short times, and know that it rolls over much sooner.

## Why it happens

millis() returns an unsigned long that wraps back to zero after about 49.7 days. Code that compares absolute times can fail at that point, and a variable that is too small, such as an int, wraps much sooner.

Store times in unsigned long variables and compare by subtracting, which stays correct across the wrap.

```
unsigned long last = 0;
if (millis() - last >= 1000UL) {   // works across the wrap
  last = millis();
}
```

Example sketch: change pins and values for your own wiring. It has not been bench-tested on your hardware.

## If you see this, do this

| You see | Do this |
| --- | --- |
| Timer stops after some seconds | Variable is an int. |
| Timer fails after weeks | Use subtraction. |

## Related

[delay and millis](https://roboticsengineering.in/fix/arduino-delay-blocks-robot-use-millis/), [integer division](https://roboticsengineering.in/fix/arduino-integer-division-returns-zero/).

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.