The AMDesignator property is used for all times from 0:00:00 (midnight) to 11:59:59.999.
If a custom format string includes the "tt" format specifier and the time is before noon, the DateTime.ToString or ToString method includes the value of the AMDesignator property in place of "tt" in the result string. If the custom format string includes the "t" custom format specifier, only the first character of the AMDesignator property value is included. You should use "tt" for languages for which it is necessary to maintain the distinction between A.M. and P.M. An example is Japanese, in which the A.M. and P.M. designators differ in the second character instead of the first character.
For cultures that do not use an A.M. designator, this property returns an empty string.
Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)