Hi all -- I'm trying to create a Process Builder flow to send an email alert if the end user changes the DATE of an event (if they change the time, but leave the date the same it should not fire).  I'm trying this formula for the criteria:

AND(NOT(ISNEW()),
OR(
DATEVALUE(PRIORVALUE([Event].StartDateTime)) != DATEVALUE([Event].StartDateTime), DATEVALUE(PRIORVALUE([Event].EndDateTime) != DATEVALUE([Event].EndDateTime)
)

When it fires I get this error:
Error element in formula: the field StartDateTime is not merge-field visible in ISCHANGED and PRIORVALUE

The process is set to fire on creation and edit, but I don't want an email when the record is first created, hence the NOT(ISNEW()).

What is the error trying to tell me?

Thanks!
Paul
 

If an email is shared to a different business unit would the converion tracking information be in the business unit that the email is deployed out of OR the business unit that the email was built in?