After some hours debugging, I found out:
The datetime return from Silverpop API is set in timezone of the login user. Why?
– login user has 1-1 relation with the refresh token (Want to change? Go to Org Amins, Oganization Settings, Application Account Access)
– refresh token has 1-1 relation with the oauth-key
– and we use oauth-key to make API call
So if we use user@examplecom/somepassword to login to https://engageXX.silverpop.com/settings.do?action=userProfile
then change the Localization/TimeZone to GMT
For example, if we call API to get all MailingTemplate, then LastModified Date of each MailingTemplate in the Response is in GMT timezone