Why Android holds your reminders back, and the Reminder check that fixes it

A reminder that arrives forty seconds late is a reminder that did not arrive. Android has three separate ways of holding one back, none of them visible from inside an app, and each phone maker adds its own. Since 2.1.20 the app can tell you which one is in the way and take you to the switch.

Three places a reminder gets held

• Battery optimisation. The default for every app. The phone batches background work and lets it run when it feels like it, which in a deep sleep can be tens of minutes later.
• Alarms and reminders. A separate permission on recent Android versions. Without it, an app may only ask for an approximate time, and the phone decides how approximate.
• The maker's own power settings. Several brands ship a second battery manager on top of Android's, with its own list of apps allowed to wake the phone. It is not in the usual Settings path and it overrides the first two.

The Reminder check

Settings has a Reminder check. It reads each of the three and says whether this phone can hold reminders back, with a button that opens the right switch for each one. Then it sends a test reminder that rings in a minute, so you can watch it work instead of trusting a green tick. Run it once on a new phone and once after a big Android update, which is when the switches tend to reset.

What a reminder does once it is allowed

• With Alarms and reminders allowed, a reminder rings on the minute. In a test on a sleeping phone, an exact alarm fired within a few milliseconds of its time; the inexact kind arrived close to a minute late.
• A reminder for a task with a time comes back once, two hours later, if you did nothing with it.
• When only one task is due, the notification carries Mark done and Snooze 1 day. Snooze keeps the task's shape: an all-day task stays all-day, a timed one keeps its time.
• The daily reminder lists every unfinished task due on your local day, as one push and one email, and each task is in it once.

Time zones, briefly

Today means today where you are. The website and the Android app tell your account which time zone they are in, and a new one is picked up the next time you open the app after travelling. There is no setting for it; until either has reported a zone, your day is counted in UTC.