how to increment a date and time they are split in to fields

I need to do a sweep of a database and increment the start date time by x amount of hours. the hours to increment is derived in an earlier statement. basically i have two fields

START_DATE datetime
START_TIME datetime

I guess i need to cast them into a single field, increment it by the increment value and write it back, I would liked to achieve this in a single statement. Can anyone suggest how this statement might look?

Thanks in advance