# Clock accuracy


> A schedule is only as good as the clock evaluating it. How screens get the time, and the one case where a player comes back wrong.
Screens evaluate their schedules locally, against their own clock. A screen whose clock is wrong runs your schedule at the wrong time, and nothing in the dashboard looks broken while it does.

## How a screen gets the time

Over the network, from a time server, using NTP on UDP port 123.

That is why NTP appears in the [network checks](/docs/screens/monitoring/health-and-diagnostics/) on a screen's Details tab, and why a failing time check is reported as a scheduling problem rather than a networking curiosity.

If outbound NTP is blocked, point the screens at an internal time server instead. See [Configure time servers](/docs/screenlyos/networking/guides/configure-time-servers/).

## The Raspberry Pi has no battery clock

A Raspberry Pi based [Screenly Player](/docs/screenly-players/) has no hardware real-time clock, so it does not keep time while powered off. On boot it has no idea what time it is until it reaches a time server.

Normally that is a few seconds and nobody notices. It matters when a player **reboots while the network is down**: it comes back with no reliable time and cannot evaluate schedules correctly until the connection returns.

The [Player Max](/docs/screenly-players/) has a real-time clock and keeps the correct time through a reboot with no network at all.

That is a genuine reason to prefer a Player Max on a site where power is less reliable than it should be.

## When the clock breaks more than scheduling

A clock wrong by more than a few minutes also breaks TLS, because certificates are only valid between two dates. A device that thinks it is 2019 cannot connect to us at all, and presents as a screen that will not come online rather than as a clock problem. See [Common setup issues](/docs/get-started/troubleshooting/setup-issues/).

## Checking a screen's time

A screen's Overview tab shows its **local time**. That is the fastest check when scheduled content appears at the wrong hour: compare it against what the clock on the wall next to it says.

A screen an hour out is usually in the wrong time zone rather than genuinely wrong. See [Time zones](/docs/scheduling/time/time-zones/).