If you send your mail merge manually

YAMM starts sending your mail merge as soon as you click Send emails. The time zone displayed in the tooltip under the Merge status column is based on the spreadsheet’s time zone.

If you schedule your mail merge

The mail merge is sent according to your browser's time zone (not the time zone of your spreadsheet). It means that your recipients that are in different time zones may receive the email at a different time than intended if you schedule it according to your time zone.

To avoid this issue, schedule the mail merge based on the majority of your recipients' time zone.

To check the time zone of your browser, select Yet Another Mail Merge in the Add-ons menu. Click Start Mail Merge.

Then click the Delay Delivery button. Next to the Delivery time field, you can see your time zone.