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<?php
 
/**
 * format date
 * 2019-06-12
 *
 * @param  string
 * @return string
 */
function format_date($date = NULL)
{
    // split string
    $element = explode('-', $date); // 2019-06-12
    $year  = (int)@$element[0]; // 2019
    $month = (int)@$element[1]; // 06
    $day   = (int)@$element[2]; // 12
    // set default value
    if ($year == 0) $year = 1;
    if ($month == 0) $month = 1;
    if ($day == 0) $day = 1;
    // format with PHP DateTime Class
    $d = new DateTime();
    $d->setDate($year, $month, $day);
    $format_date = $d->format('Y-m-d');
    return $format_date;
}
?>

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<?php
 
/**
 * format date
 * 2020-05-17
 *
 * @param string $date
 * @return string $date_format
 */
function format_date($date = "")
{
    # UTC : split date + "T" + time
    $element = explode('T', $date); # 1976T => [1976, T]
    $date_part = $element[0]; # 1976
 
    # 年月日: replace to "-" 1976年8月1日 => 1976-8-1-
    $unit = array("年", "月", "日");
    $date_part = str_replace($unit, "-", $date_part);
 
    # split Y + m + d
    $YMD = explode('-', $date_part);
    $year  = (int) @$YMD[0]; # 1976
    $month = (int) @$YMD[1]; # 08
    $day   = (int) @$YMD[2]; # 01
 
    # 数字以外を除去 1976年 → 1976
    $year = preg_replace('/[^0-9]/', '', $year);
    $month = preg_replace('/[^0-9]/', '', $month);
    $day = preg_replace('/[^0-9]/', '', $day);
 
    # default date
    $year_start = 1;
    $month_start = 1;
    $day_start = 1;
    if ($year < 1) {
        $year = $year_start;
    }
    if ($month < 1) {
        $month = $month_start;
    }
    if ($day < 1) {
        $day = $day_start;
    }
 
    # format
    $format = "Y-m-d";
    $date_comp = $year . "-" . $month . "-" . $day; # compose Y-m-d
    $d = DateTime::createFromFormat($format, $date_comp);
    $date_format = $d->format($format);
 
    return $date_format;
}

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<?php
 
/**
 * validation datetime format
 *
 * @param string $date
 * @param string $format
 * @return bool
 */
function validateDate($date, $format = 'Y-m-d H:i:s')
{
    $d = DateTime::createFromFormat($format, $date);
    return $d && $d->format($format) == $date;
}