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