What alternative approach can be used to calculate the timestamp for a specific date before the Unix timestamp started in 1970?

Before the Unix timestamp started in 1970, an alternative approach to calculate the timestamp for a specific date is to use a different epoch reference point. One common epoch reference point used is the Julian Day Count, which is the count of days since the beginning of the Julian Period on January 1, 4713 BCE. By converting the specific date to its corresponding Julian Day Count, you can then calculate the timestamp based on that reference point.

// Function to calculate timestamp from a specific date before 1970 using Julian Day Count
function getTimestampFromDate($year, $month, $day) {
    $julianDayCount = gregoriantojd($month, $day, $year);
    $timestamp = ($julianDayCount - 2440587.5) * 86400; // Convert Julian Day Count to timestamp
    return $timestamp;
}

// Usage example
$year = 1000;
$month = 1;
$day = 1;
$timestamp = getTimestampFromDate($year, $month, $day);
echo "Timestamp for $day/$month/$year before 1970: $timestamp";