You will want to configure a Field Calculator whose Expression looks something like substring(DateAsString,5,7) to pull a two character substring out of a date/time field received as a String and write the retrieved sub-string to a new field. The index values 5 and 7 in the substring( ) syntax specify the starting index and ending index of a zero-based array of characters.
The downside of this is that the date/time value must be specified, by the GeoEvent Definition, as a String and the service elements in the GeoEvent Service will only be able to work with the date/time as a String ... not a Date. You can work around this by feeding the event data back into a second input and allowing the second input to handle the conversion of the String representation of the date/time to an actual Date value.
Details are included in the attached PDF.
Please note that support for Java String functions from within a Field Calculator is supported in the 10.2.2 release of GeoEvent Processor, which should be publically available on your Customer Care Portal tomorrow 15-April-2014. Esri Distributors should have been given access to download the new release last week.
- RJ