SharePoint Online - Send e-mail alert to notify Item or Document expiration Office 365

This can be done by an easy trick using SharePoint designer

Here are the high-level steps

1. Create a calculated column to calculate the Expiry Date.

2. Create a SPD workflow that starts upon item creation.

3. Build a variable "Notification date" of the type date, that stores the value of "Expiry Date" plus or minus number of days before or after when you want to send the notification. For example if you want to send the notification a day before the item is going to expire then say "Expiry Date - 1 day"

3. Next add a step to pause the workflow until the current date is equal to "Notification date". When the system date becomes equal to "Notification date" the workflow can be set to send a custom e-mail message.

 
You would write something like below

Add -1 days to List:Expiry Date (Output to Variable:Date)
then Pause until Variable:Date
then Send email toOwner