PhpSpreadsheet/tests/data/Shared/Date/DateTimeToExcel.php

<?php

// DateTime object                          Result           Comments
return [
    [new DateTime('1900-01-01'),           1.0],    //  Excel 1900 base calendar date
    [new DateTime('1900-02-28'),           59.0],    //  This and next test show gap for the mythical
    [new DateTime('1900-03-01'),           61.0],    //      MS Excel 1900 Leap Year
    [new DateTime('1901-12-14'),           714.0],    //  Unix Timestamp 32-bit Earliest Date
    [new DateTime('1903-12-31'),           1461.0],
    [new DateTime('1904-01-01'),           1462.0],    //  Excel 1904 Calendar Base Date
    [new DateTime('1904-01-02'),           1463.0],
    [new DateTime('1960-12-19'),           22269.0],
    [new DateTime('1970-01-01'),           25569.0],    //  Unix Timestamp Base Date
    [new DateTime('1982-12-07'),           30292.0],
    [new DateTime('2008-06-12'),           39611.0],
    [new DateTime('2038-01-19'),           50424.0],    //  Unix Timestamp 32-bit Latest Date
    [new DateTime('1903-05-18 13:37:46'),  1234.56789],
    [new DateTime('1933-10-18 16:17:37'),  12345.6789],
    [new DateTime('2099-12-31'),           73050.0],
];