def merge_date(df, year='Year', month='Month', day='Day', hours='Hours',
    seconds='Seconds'):
    """
    * Function: merge_date 
    * Usage: merge_date(DataFrame, col_year, col_month, col_day) . . .
    * -------------------------------
    * This function returns Datetime in the format YYYY-MM-DD from
    * input of a dataframe with columns holding 'Year', 'Month', 'Day' 
    """
Convert date/time columns in Pandas dataframe