How to concatenate datetime type field and a float type field. After that parse as a datetime datatype field

My teacher told me to select columndate cocat columntime as column3, then parse column3 as datetime datatype. He says I can't change the database datatype and I need to use those 4 column field start_date, end_date, start_time and end_time as it's in a server so he told me to do this.
He meant to combine start_date field and start_time field as one column in the code, then convert it to datetime type also done in the code. Same goes for end date field and end_time field. That means for example, start_date and start_time for record 1 will become 3/4/2014 9:00:00AM, end_date and end_time for record 1 will become to 3/4/2014 12:00:00PM.
so below are my codes, it's for a scheduler/timetable, which is called daypilot. So can help me in the query for the converting? in the code below there is a start_date and end_date, that is to be replaced by the ...