Add functions for transferring dates to ints and vice versa

src/MD_STD.php

@@ -928,4 +928,58 @@ final class MD_STD {
         return $output;
 
     }
+
+    /**
+     * Transfers a date into an integer. 0000-01-01 > 101; 2001-01-01 > 20010101.
+     * Needed to store negative dates (BC) in MySQL.
+     *
+     * @param string $date Date.
+     *
+     * @return integer
+     */
+    public static function date_to_int(string $date):int {
+
+        // Edge case: -0000-01-01
+        if (substr($date, 0, 5) === '-0000') {
+            return -1 * intval(str_replace("-", "", ltrim($date, "0-")));
+        }
+
+        return intval(date("Ymd", strtotime($date)));
+
+    }
+
+    /**
+     * Transfers an integer into a date, reversing the effects of date_to_int.
+     * 0000-01-01 > 101; 2001-01-01 > 20010101.
+     * Needed to retrieve negative dates (BC) stored in MySQL.
+     *
+     * @param int $date_int Date represented as an integer.
+     *
+     * @return string
+     */
+    public static function int_to_date(int $date_int):string {
+
+        $dateStr = (string)$date_int;
+
+        if (substr($dateStr, 0, 1) === '-') {
+            $isNegative = true;
+            $dateStr = substr($dateStr, 1);
+        }
+        else $isNegative = false;
+
+        if (strlen($dateStr) < 8) {
+            $dateStr = str_pad($dateStr, 8, "0", STR_PAD_LEFT);
+        }
+
+        $day = substr($dateStr, -2, 2);
+        $month = substr($dateStr, -4, 2);
+        $year = substr($dateStr, -8, 4);
+
+        return match($isNegative) {
+            true => '-',
+            false => '',
+        } . $year . '-' . $month . '-' . $day;
+
+
+    }
 }