Greg,
Sorry for the trouble with the dates. We'll get the expected date format documented soon.
The date format should be in this format. All date/times are in UTC/Zulu timezone.
YYYY-MM-DDThh:mm:ssZ
YYYY is the year.
MM is the month.
DD is the day of the month.
hh is the hour of the day as on a 24-hour clock.
mm is minutes.
ss is seconds.
Z is a literal 'Z' character indicating that this string representation of the date is in UTC
I tested a test with filtering on the animals.updatedDate and it's working without errors.
Hope that helps.
Richard