def flatten(lst):
    return [item for sublist in lst for item in sublist]


def timestamp_fields(field, prefix=''):
    """ Find all fields with Timestamp metadata """
    if isinstance(field, StructType):
        return flatten(timestamp_fields(f, prefix) for f in field.fields)

    if isinstance(field, StructField):
        if field.metadata and field.metadata.get('encoding') == 'timestamp':
            return [prefix + '.' + field.name if prefix else field.name]
        return timestamp_fields(field.dataType, field.name)

    return []


fields = timestamp_fields(df.schema)