I use Google Calendar to sync across devices, but not the app from Google itself
In ANDROID:
Syncing with Google Calendar I use Today Calendar. Bought Business Calender 2 also, but have that not installed anymore. Like Today more. But praticly every Calendar app for Android can sync with Google Calendar.
In WINDOWS 10:
-the official Calendar app from Microsoft syncs my Google Calendar without issue. I have ave Outlook caendar disabled. (Click menu (hamburger), enable Gmail, disable Microsofte account. Must have Gmail account added to Mail app in Windows 10)
-One of the nicest email apps for Windows syncs Google Calendar (and Gtasks) without issue here too: EM Client (free maximum 2 email accounts, can enable touch UI): http://www.emclient.com/homepage?campaign=36&utm_campaign=CJ
-Thunderbird ( https://www.mozilla.org/en-US/thunderbird/ ) can sync with Google Calendar with addon , see: https://support.mozilla.org/nl/kb/lightning-gebruiken-met-google-agenda?cache=no . The Addon needed is Provider for Google Calendar ( https://addons.mozilla.org/nl/thunderbird/addon/provider-for-google-calendar/ ). Enable month view in Thunderbird: When in Mail tab, use View-> Today Pane -> Show Mini-Month