In spreadsheets, dates have a native value of an integer representing the number of days since december 31 1899 and time is a decimal value which is the fraction of a day ( 6 hours = 1/4 of a day for example , 0.25 day).
So when you add DATE+TIME (integer+decimal) in a spreadsheet you get a full date with time .
So the answer (as you noticed it in your comment on the other answer) is logically to ADD both values. That's actually the reason spreadsheets are build like that ! (to make it easy to use date and time)
Use the formula =D2+E2
in a new column and you get a complete date object directly useable in JavaScript.
In Javascript date and time are the same objects, there is no time object that has no date and no date without time : their native values are milliseconds counted from January 1 1970 at midnight (which is an integer).