Below solution is for people who use standalone (no Apache etc.) PHP 5.2.17, 5.3.25 with IIS 7.5 / 8 ONLY
I used to have this trouble for 3 days straight, looking all over but no help, if you set the date.timezone in the php.ini and still no luck, make sure the PHP version you use is Threat Safe, even though PHP.net recommend the IIS server user use non-Threat Safe version.
I have setup dozens of server using PHP Threat Safe on all the PHP-based application, trust me, non-Threat Safe version on IIS will even cost the extension not to respond to the PHP application, in this case, Roundcube Mail.
Best install solution, PHP 5.3.25 with MySQL 5.6.11 (newest as of May 27th, 2013) guarantee no problem...