Problem:

The date picker was displaying a date one day earlier than the date selected in the calendar (e.g., selecting Feb 4 showed Feb 3 in the input field), even though the WordPress timezone was set correctly. The issue occurred both on localhost and on the production site.

Solution:

The issue was caused by a global date_default_timezone_set('Asia/Manila') call introduced by the QuickBooks PHP SDK. Setting the PHP default timezone to a non-UTC value affected the JavaScript/PHP date handling used by the date picker, resulting in an off-by-one-day shift. Removing this call or keeping the default timezone as UTC resolved the issue and restored correct date selection behavior.