The example below used two fields; one named “DatePickerField” and the other “TimeSelectBox”. Create a third textbox named DateTimeGroup (or whatever you want) and change the data type to “Date Time”. In the default value box type:
concat(DatePickerField, “T”, TimeSelectBox)
Whenever you choose a date and time the DateTimeGroup box will automatically be populated based on what date and time was chosen. If you want to hide the DateTimeGroup field on the form just set the size to zero and the data it collects can still be used in a SharePoint form library.