To some extent, you can convert a value from one temporal type to another. However, there may be some alteration of the value or loss of information. In all cases, conversion between temporal types is subject to the range of valid values for the resulting type. For example, although DATE, DATETIME, and TIMESTAMP values all can be specified using the same set of formats, the types do not all have the same range of values. TIMESTAMP values cannot be earlier than 1970 UTC or later than '2038-01-19 03:14:07' UTC. This means that a date such as '1968-01-01', while valid as a DATE or DATETIME value, is not valid as a TIMESTAMP value and is converted to 0.

Conversion of DATE values:

Conversion of DATETIME and TIMESTAMP values:

For conversion of TIME values to other temporal types, the value of CURRENT_DATE() is used for the date part. The TIME is interpreted as elapsed time (not time of day) and added to the date. This means that the date part of the result differs from the current date if the time value is outside the range from '00:00:00' to '23:59:59'.

Suppose that the current date is '2012-01-01'. TIME values of '12:00:00', '24:00:00', and '-12:00:00', when converted to DATETIME or TIMESTAMP values, result in '2012-01-01 12:00:00', '2012-01-02 00:00:00', and '2011-12-31 12:00:00', respectively.

Conversion of TIME to DATE is similar but discards the time part from the result: '2012-01-01', '2012-01-02', and '2011-12-31', respectively.

Explicit conversion can be used to override implicit conversion. For example, in comparison of DATE and DATETIME values, the DATE value is coerced to the DATETIME type by adding a time part of '00:00:00'. To perform the comparison by ignoring the time part of the DATETIME value instead, use the CAST() function in the following way:

date_col = CAST(datetime_col AS DATE)

Conversion of TIME and DATETIME values to numeric form (for example, by adding +0) depends on whether the value contains a fractional seconds part. TIME(N) or DATETIME(N) is converted to integer when N is 0 (or omitted) and to a DECIMAL value with N decimal digits when N is greater than 0:

mysql> SELECT CURTIME(), CURTIME()+0, CURTIME(3)+0;
+-----------+-------------+--------------+
| CURTIME() | CURTIME()+0 | CURTIME(3)+0 |
+-----------+-------------+--------------+
| 09:28:00  |       92800 |    92800.887 |
+-----------+-------------+--------------+
mysql> SELECT NOW(), NOW()+0, NOW(3)+0;
+---------------------+----------------+--------------------+
| NOW()               | NOW()+0        | NOW(3)+0           |
+---------------------+----------------+--------------------+
| 2012-08-15 09:28:00 | 20120815092800 | 20120815092800.889 |
+---------------------+----------------+--------------------+