udm fields -start date and discovered date

More context here please.  
screenshot?  

Here is a link to our UDM field list, which contains a breakdown of each field.  Like @dnehoda mentioned, if you're able to provide extra context around the use case we can help out further.

Unified Data Model field list 

This is likely originating from a multi-event rule. Could you please check the rule conditions? If possible, share the details with us so we can assist further in understanding.