from datetime import datetime


def to_date_string(value):
    """
    gets the date string representation of input with default format.

    for example: `2015-12-24`

    :param datetime | date value: input object to be converted.

    :rtype: str
    """

    date = value
    if isinstance(value, datetime):
        date = value.date()

    return date.isoformat()