How do we know when the SharePoint site was last updated?

We have multiple “when the site was modified last time” properties – e.g. some we can retrieve with SharePoint CSOM:

• Site LastContentModifiedDate
• Web LastItemModifiedDate
• Web LastItemUserModifiedDate

Also we can get MS Graph site LastModifiedDateTime property and use “Last activity” field via Admin Center GUI

And we can modify site in a multiple ways – update document or list item on the site, change library or site settings, configure site permissions, assign site sensitivity label, setup site property and so on.

Question: which site “last modified” properties reflect what events/actions?

This might be important if we think of retention policies, or any kind of clean-up processes… Let say, we are getting report on abandoned sites (inactive sites), but we are also assigning sites sensitivity labels, or we are updating site custom properties (e.g. for adaptive scopes), we have an ownerless groups policy working etc.

What if we assign site sensitivity label to an inactive (5 years old site) – would it affect site retention since site was updated?

Results

So i did some tests and based on detailed results below, it seems like

• Web LastItemModifiedDate is triggered when user visited site (but not LastItemUserModifiedDate)
• If a document or list Item updated by user or app – all properties are triggered
• MS Graph site property LastModifiedDateTime, root web property LastItemModifiedDate and Site LastContentModifiedDate – same values
• If site custom property is updated – it does not affect any site “last modified” property
• The same for sensitivity label updated by app – it does not affect any site “last modified” property
• The same for Microsoft ownerless groups policy – when user accept or decline group membership – no site “last modified” properties are changed (the same is true for Microsoft 365 group last modified date/time property).

Please refer to the table below

Detailed test results

Test results if the event triggers property update:

Event	Last Content Modified Date	Last Item Modified Date	Last Item User Modified Date	Graph Last Modified DateTime	GUI Last activity
Page viewed by user	Yes	Yes	No	Yes
Home Page viewed by user
Site Page viewed by user
Document or list item updated by user	Yes	Yes	Yes	Yes
Document or list item updated by app	Yes	Yes	Yes	Yes
Site config settings updated by user
Site config settings updated by app
Site custom property updated by app	No	No	No	No
Site Sensitivity label updated by user via SharePoint	Yes	No	No	No
Site/Group Sensitivity label updated by user via Teams
Site/Group Sensitivity label updated by user via Azure	No	No	No	No
Site Sensitivity label updated by app	No	No	No	No
Site collection admin updated by user	Yes	Yes	No	Yes
Site collection admin updated by app	Yes	Yes	No	Yes
SharePoint group membership updated by user	Yes	Yes	No	Yes
Standalone Site connected to a group by user	Yes	Yes	Yes	Yes
Add Microsoft Teams to Site by User	Yes	Yes	Yes	Yes
Update m365 group membership via M365 admin console by admin	Yes	Yes	No	Yes
Update m365 group membership via Azure by admin
Update m365 group membership via Teams by user	No	No	No	Yes
Update m365 group membership via App
Accept group ownership invitation sent by ownerless groups policy	No	No	No	No
Decline group ownership invitation sent by ownerless groups policy	No	No	No	No