When you create a new Office 365 tenant, all user mailboxes will have the default timezone and language. In my case, I work in the Netherlands, the preference for most companies is to set the Time zone to Central European Time (GMT +1) and the language of the users default folders to Dutch.
You can either ask the users to logon to webmail using https://outlook.office.com and fill in the first time question to set the time zone and default language. But how cool would it be to do this for all your users using PowerShell?
When you do large migrations, it might be convenient to change the default mailbox language settings for all your end users. By default each user needs to set the default language and time zone at first login to OWA in Office 365.
With the following PowerShell Script you should be able to change it within a few seconds. In this script we used the Dutch language code and Western European Standard time. Change it accordingly.
get-mailbox | Set-MailboxRegionalConfiguration -LocalizeDefaultFolderName: $true -DateFormat dd-MM-yy -Language 1043 -TimeZone "W. Europe Standard Time"