You appear to be working with a few misconceptions.
TIME_FORMAT is just a format string. Functions are not processed, but are considered part of the time string. TIME_PREFIX should be the text that comes before the timestamp. It is also a literal string. TIMESTAMP_FIELDS doesn't apply since you are not using INDEXED_EXTRACTIONS.
Try these settings:
TIME_PREFIX="DATE" : "
--- If this reply helps you, an upvote would be appreciated.