If you are interested to know when your mailbox is created, try below PowerShell Oneliner…Basically what this does is, it uses Outlook application and fetches one of it’s schema property named PR_CREATION_TIME which contains the mailbox creation time.
(New-Object -ComObject Outlook.Application).Session.DefaultStore.GetRootFolder().PropertyAccessor.GetProperty(http://schemas.microsoft.com/mapi/proptag/0x0E580102)
[Source:Shavy Blog]