In ArcGIS Pro 2.6 it seems that you can format date data that is inserted as dynamic text using the information in this page: Format date and time dynamic text—ArcGIS Pro | Documentation however this only seems to apply to dynamic text inserted using options like the Current time or Date exported.
I have a date field in my index layer that I only want to display as dd/mm/yy, but if I apply the format option it does nothing.
Date Exported: <dyn type="layout" name="Base Component Layout" property="dateExported" format="short|short"/>
<dyn type="page" property="attribute" field="rev_date"/>
It isn't mentioned in this article at all: Using dynamic text with Data Driven Pages—Help | ArcGIS for Desktop It seems like it is not possible to format data attributes that are used as dynamic text. Can someone from Esri please confirm.
If you are pulling an attribute from the index layer (Map Series > Attribute) you will not be able to format the attribute value. Think of this option as the quick and simple way to add dynamic text from an index layer.
You can format an attribute value if you use the Table Attribute > Value dynamic text element. This dynamic text element is much more robust than the Map Series > Attribute element.
With this element you can connect to a (Spatial) Map Series...
... and use Arcade to format values from a Date field.
Please see Add and modify dynamic text—ArcGIS Pro | Documentation (look for "Table dynamic text") and Date Functions | ArcGIS for Developers (Arcade help) for more information.
Hope this helps,