Such calculations may lead to unpredictable results because when a date is represented as a MySQL string, MySQL attempts to perform the string execution on only the first occurrence of the string. Number crunching. The following example will clarify it -
mysql> select '2017-10-17' + 20; +-------------------+ | '2017-10-17' + 20 | +-------------------+ | 2037 | +-------------------+ 1 row in set, 1 warning (0.00 sec) mysql> select '2017-10-25' - 17; +-------------------+ | '2017-10-25' - 17 | +-------------------+ | 2000 | +-------------------+ 1 row in set, 1 warning (0.00 sec) mysql> select '2017-10-17' * 2; +-------------------+ | '2017-10-17' * 20 | +-------------------+ | 4034 | +-------------------+ 1 row in set, 1 warning (0.00 sec) mysql> select '2017-05-25'/5; +----------------+ | '2017-05-25'/5 | +----------------+ | 403.4 | +----------------+ 1 row in set, 1 warning (0.00 sec)
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