Add Doing Date Math as a mysql til

mysql/doing-date-math.md

@@ -0,0 +1,31 @@
+# Doing Date Math
+
+MySQL has an array of functions for interacting with `date` and `datetime`
+values. If you'd like to do math with a date to compute a date in the future
+or the past, you can use the
+[`DATE_ADD()`](https://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html#function_date-add)
+and
+[`DATE_SUB()`](https://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html#function_date-sub)
+functions.
+
+```mysql
+mysql> select now() Now, date_add(now(), interval 10 minute) '10 Minutes Later';
++---------------------+---------------------+
+| Now                 | 10 Minutes Later    |
++---------------------+---------------------+
+| 2018-10-18 15:53:29 | 2018-10-18 16:03:29 |
++---------------------+---------------------+
+
+mysql> select now() Now, date_sub(now(), interval 9 day) '9 Days Earlier';
++---------------------+---------------------+
+| Now                 | 9 Days Earlier      |
++---------------------+---------------------+
+| 2018-10-18 15:54:01 | 2018-10-09 15:54:01 |
++---------------------+---------------------+
+```
+
+There are equivalent `ADDDATE()` and `SUBDATE()` functions if you prefer.
+
+Check out [the
+docs](https://dev.mysql.com/doc/refman/5.5/en/date-and-time-functions.html)
+for more details.