Hi praga,
I would like to confirm the following things:
1. Do you mean the email date is not correct at OWA(mail.office365.com), but it is correct in the Mac mail client?
2. Which Office 365 plan do you subscribe to?
To troubleshoot the issue, could you make sure you have set the correct time zone in Mac and OWA option? If the issue persists, to better understand the current situation, could you provide the screenshot of the issue for further investigation?
Thanks,
Johnny Zhang