The item details page for an item in ArcGIS Online shows the date of the last updates for an item. There are two update dates listed on this page and each traces certain types of updates.
The Updated field at the top of the item details page (next to the thumbnail) tracks the updates made to the item in ArcGIS Online, for example, the name of the item or layers, description, metadata, and so forth.
Under the Details section on the right side of the pane, the Data Last Updated date shows the last update made to the content of a data or the schema, for example, editing an attribute, adding a feature, adding a new field, etc.
Some changes update both fields. For example, changing the layer style from the Visualization tab or enabling and/or disabling editing (because it also changes the attribute indexes).
However, there are other changes that apparently are not tracked in either field. For instance, changing the ownership or sharing settings of an item.