Fresh upgrade from 1.9.14 to 1.9.15 to v1.10 (901e5ea), running on IIS 8.0 (PHP v5.6.24; MySQL 5.7.14).
Setting the Date and Time Format to "Locale Defaults", sets the time to 24-hour format.
Trying to set Advanced, blanks out most date/time values in the system.