FDateTime is not correctly formatted by As Date Time when using any dates before January 1st, 1970

FDateTime is not correctly formatted by As Date Time when using any dates before January 1st, 1970.

From a conversation on Slack with MaxP:
this is probably an underflow
the ICU based string formatter converts the FDateTime to a unix timestamp, which is then formatted as a string
so you probably can't format any dates before January 1st 1970, because that's when the Unix epoch starts
that's a bug for the localization team

Reproduced in 4.7 branch from GitHub (4.7.4) and in Main (//depot/UE4/Promotable-CL-2495212)

1. Open any project
2. Create a new Actor Blueprint
3. Add a DateTime variable
4. Leave the default value (0001.01.01-00.00.00)
5. Get the DateTime variable > As Date Time > Print Text
6. Attach to Event Begin Play
7. PIE

Result:
Print String returns: "Jun 7, 1906, 1:35:44 PM". If the default is set to anything after 01/01/1970, the date/time is shown correctly (UTC-5h).