Combining 2 strings, and converting to a Date / Time field?

Combining 2 strings, and converting to a Date / Time field?

I have an application where a user selects a report date, and then a starting time.  I have a date field for the report date, and a drop down menu with 15 minute increments for the starting time.  I am trying to combine these two fields, and then place them in a Date / Time field, but I can not get it formatted right.  Here is the code I currently have.

datetime_string = input.report_date + input.start_time_input;
input.start_time = datetime_string.toTime("MM/dd/yyyy");

Which gives me the result below in the Date / Time field "start_time"

05/15/201406 00:00:00

The times in the drop down are formatted as 06:00 , 06:15, 06:30 etc...

How can I accomplish this and have it properly formatted?

Thank you!