php - Should we put a timer on server or mobile?

Solution:

Use aqueue system on the server. Create a queue for sending emails.

When you want to send an email just push it in the queue, with time or DateTime in your case. Then run aCron Job to check, process and send the email.

Also, make sure to implement the retry functionality, so that even email sending was failed the Cron will take the job again and try to send for anumber of times.

These Links will Help:

Managing Cron Jobs in PHP

Setting Up Cron in Ubuntu

Setting up Cron using cPanel

If you are confused about how to setup the Timings of Cron, checkout this website:

CronTab Guru

Solution:

This is better to implement the timer on your server. The negative possibility that would happen and block the function if you implement the timer on the device is when the device is on power-off mode or the device couldn't connect to the server to trigger for sending the email.

The serverinstance is should long-lived, this is should never go down, and this could be more reliable for your case, as @Harish ST answer, you can take a profit from cron job / cron task.