xlsxwriter.class.php
public static function convert_date_time($date_input) //thanks to Excel::Writer::XLSX::Worksheet.pm (perl)
	{
		$days    = 0;    # Number of days since epoch
		$seconds = 0;    # Time expressed as fraction of 24h hours in seconds
		$year=$month=$day=0;
		$hour=$min  =$sec=0;

		$date_time = $date_input;
		if (preg_match("/(\d{4})\-(\d{2})\-(\d{2})/", $date_time, $matches))
		{
			list($junk,$year,$month,$day) = $matches;
		}
		if (preg_match("/(\d+):(\d{2}):(\d{2})/", $date_time, $matches))
		{
			list($junk,$hour,$min,$sec) = $matches;
			$seconds = ( $hour * 60 * 60 + $min * 60 + $sec ) / ( 24 * 60 * 60 );
		}

		//using 1900 as epoch, not 1904, ignoring 1904 special case

		# Special cases for Excel.
		if ("$year-$month-$day"=='1899-12-31')  return $seconds      ;    # Excel 1900 epoch
		if ("$year-$month-$day"=='1900-01-00')  return $seconds      ;    # Excel 1900 epoch
		if ("$year-$month-$day"=='1900-02-29')  return 60 + $seconds ;    # Excel false leapday

		# We calculate the date by calculating the number of days since the epoch
		# and adjust for the number of leap days. We calculate the number of leap
		# days by normalising the year in relation to the epoch. Thus the year 2000
		# becomes 100 for 4 and 100 year leapdays and 400 for 400 year leapdays.
		$epoch  = 1900;
		$offset = 0;
		$norm   = 300;
		$range  = $year - $epoch;

		# Set month days and check for leap year.
		$leap = (($year % 400 == 0) || (($year % 4 == 0) && ($year % 100)) ) ? 1 : 0;
		$mdays = array( 31, ($leap ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 );

		# Some boundary checks
		if($year < $epoch || $year > 9999) return 0;
		if($month < 1     || $month > 12)  return 0;
		if($day < 1       || $day > $mdays[ $month - 1 ]) return 0;

		# Accumulate the number of days since the epoch.
		$days = $day;    # Add days for current month
		$days += array_sum( array_slice($mdays, 0, $month-1 ) );    # Add days for past months
		$days += $range * 365;                      # Add days for past years
		$days += intval( ( $range ) / 4 );             # Add leapdays
		$days -= intval( ( $range + $offset ) / 100 ); # Subtract 100 year leapdays
		$days += intval( ( $range + $offset + $norm ) / 400 );  # Add 400 year leapdays
		$days -= $leap;                                      # Already counted above

		# Adjust for Excel erroneously treating 1900 as a leap year.
		if ($days > 59) { $days++;}

		return $days + $seconds;
	}
public static function convert_date_time($date_input) //thanks to Excel::Writer::XLSX::Worksheet.pm (perl)
	{
		$days    = 0;    # Number of days since epoch
		$seconds = 0;    # Time expressed as fraction of 24h hours in seconds
		$year=$month=$day=0;
		$hour=$min  =$sec=0;

		$date_time = $date_input;
		if (preg_match("/(\d{4})\-(\d{2})\-(\d{2})/", $date_time, $matches))
		{
			list($junk,$year,$month,$day) = $matches;
		}
		if (preg_match("/(\d+):(\d{2}):(\d{2})/", $date_time, $matches))
		{
			list($junk,$hour,$min,$sec) = $matches;
			$seconds = ( $hour * 60 * 60 + $min * 60 + $sec ) / ( 24 * 60 * 60 );
		}

		//using 1900 as epoch, not 1904, ignoring 1904 special case

		# Special cases for Excel.
		if ("$year-$month-$day"=='1899-12-31')  return $seconds      ;    # Excel 1900 epoch
		if ("$year-$month-$day"=='1900-01-00')  return $seconds      ;    # Excel 1900 epoch
		if ("$year-$month-$day"=='1900-02-29')  return 60 + $seconds ;    # Excel false leapday

		# We calculate the date by calculating the number of days since the epoch
		# and adjust for the number of leap days. We calculate the number of leap
		# days by normalising the year in relation to the epoch. Thus the year 2000
		# becomes 100 for 4 and 100 year leapdays and 400 for 400 year leapdays.
		$epoch  = 1900;
		$offset = 0;
		$norm   = 300;
		$range  = $year - $epoch;

		# Set month days and check for leap year.
		$leap = (($year % 400 == 0) || (($year % 4 == 0) && ($year % 100)) ) ? 1 : 0;
		$mdays = array( 31, ($leap ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 );

		# Some boundary checks
		if($year < $epoch || $year > 9999) return 0;
		if($month < 1     || $month > 12)  return 0;
		if($day < 1       || $day > $mdays[ $month - 1 ]) return 0;

		# Accumulate the number of days since the epoch.
		$days = $day;    # Add days for current month
		$days += array_sum( array_slice($mdays, 0, $month-1 ) );    # Add days for past months
		$days += $range * 365;                      # Add days for past years
		$days += intval( ( $range ) / 4 );             # Add leapdays
		$days -= intval( ( $range + $offset ) / 100 ); # Subtract 100 year leapdays
		$days += intval( ( $range + $offset + $norm ) / 400 );  # Add 400 year leapdays
		$days -= $leap;                                      # Already counted above

		# Adjust for Excel erroneously treating 1900 as a leap year.
		if ($days > 59) { $days++;}

		return $days + $seconds;
	}
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