from datetime import datetime


def get_start_date(product):
    """
    Define start date of product version.

    Parameters
    ----------
    product : string
        product specification

    Returns
    -------
    datetime : datetime.datetime
        timestamp of start date

    """
    dt_dict = {'M2T1NXLND.5.12.4': datetime(1980, 1, 1)}
    return dt_dict[product]