Storage Explorer Version: 1.2.0
Platform/OS Version: Win 10
Architecture: ia32
Build Number: 20180605.1
Commit: f8373d5a
Add table to any storage account (Tested on Storage Emulator 5.5 & Live storage account)
Add entity to table
Add Property and change Type to DateTime and try to enter date
Date entered in ISO 8601 format to be allowed (as per https://docs.microsoft.com/en-us/rest/api/storageservices/formatting-datetime-property-values)
Red validation border stays in field no matter the format of the input, and the Insert button is disabled.

Add a milliseconds piece: 2009-03-18T04:25:03.000Z. We can make that more clear in the validation in future versions.
I would guess this is a fairly low-priority product for Microsoft, but could this someday be updated to open a date-picker when editing a DateTime field?
Date pickers are...problematic, we've found. We struggled for a while with data accuracy and integrity problems in the past, which is why we went with an ordinary text input. The built in ones also don't let you select milliseconds or time zones.
Most helpful comment
Add a milliseconds piece:
2009-03-18T04:25:03.000Z. We can make that more clear in the validation in future versions.