fieldformat command only affects how the data is displayed by web UI elements. It does not modify the data itself. When you send via email, you're sending the data. You will need to use
eval instead of
fieldformat if you want to change the sent data.
This is confusing because when you Export the report results from the UI, the date format is preserved, but when you let Splunk email it on a schedule, the date format is lost.
It should at least be made consistent... As it stands, it's impossible to test report output without scheduling a report and reviewing the email.