This script first finds sites in specified site collection. It iterates through each site in the site collection and finds Last Modified date for sites in SharePoint online/Office 365. I was confused initally whether it's site modified date or content modified date. finally after my couple of testing scenarions i concluded this modified date is not site modified date. It's item modified date in the site which i was looking for. The results ( each site URL and last modified date) have been written and saved in csv file in specified path . You can save this file in your local drive and can be executed using powershell ISE.