Use case: to convert DateTime field (copy from Salesforce) to Date field, or Text field in Einstein Analytics.
In this blog, I'll add new fields in Dataflow using computeExpression. We will use a DateTime field with API name: Submitted_Date_Time__c
This formula will take the first 10 characters.
Original: 2017-05-03T09:43:28.000Z --> 2017-05-03
Remember to enter "Date Format" (you need to scroll down to find it), otherwise, you can't upload the Dataflow.
Side note: toDate() is case-sensitive.