def dategetter(date_property, collection):
    """
    Attempts to obtains a date value from a collection.

    :param date_property: property representing the date
    :param collection: dictionary to check within

    :returns: `str` (ISO8601) representing the date. ('..' if null or "now",
        allowing for an open interval).
    """

    value = collection.get(date_property, None)

    if value == 'now' or value is None:
        return '..'

    return value.isoformat()