If you need to find out the date of the last time a user changed their password in Office 365, this can be completed using PowerShell.
First you will need to download and connect to the Azure Active Directory Module, I created an article on this a while ago you can check that out here.
Once you have connected to the Azure AD Module (admin account) you are ready to start and should look something like this
Run the cmdlet below to get a list of all users dates of last password change
Get-MsolUser -All | select DisplayName, LastPasswordChangeTimeStamp
If you want to use this information in Excel you can simply export it to a CSV using this cmdlet:
Get-MsolUser -All | select DisplayName, LastPasswordChangeTimeStamp | Export-CSV LastPasswordChange.csv -NoTypeInformation