When the scheduler job runs next time, it will set the schedule for that row as per the modified time. If you would like to update the send time of an email that is already in the schedule queue, just change the date and time for that row in the Scheduled Date column. If the schedule date cell is empty for a row, the email recipient in that row is sent the email immediately when merge is run. When you run merge, the Mail Merge Status column of the scheduled email will be updated to reflect the number of hours or days between the current time and the scheduled date. It is also possible to send some emails immediately when merge is run and others can be scheduled. You can now specify the send date and time in the Scheduled Date column for emails that should be sent later. Once the date time format has been applied to the column, you can enter dates in short format - like Aug 2 3:30 pm and they'll converted into the correct date+time format based on your locale. To change the column format, select the entire column, go to the Format menu, choose Number and then choose Date time. This column specifies the date and time when the emails should be sent and it should be formatted so it displays both the date and time of schedule. When you configure merge, it adds a Scheduled Date column in the sheet. Select your timezone and save your settings. Open the Google Sheet, go to the File Menu and choose Spreadsheet Settings. The email scheduler uses the timezone of your Mail Merge sheet to send email.

You can also specify the send time for individual emails in a campaign and they'll be sent as per schedule. Mail Merge includes a built-in email scheduler to help you send campaigns at a later date and time automatically. How to Schedule Emails with Mail Merge and Send Later

How to set up mail merge template