How do I handle dates from a spreadsheet like this. 
I need to get the last date from column 2 and then find the next date. I have created this code for retrieving the last date.
lastDateRow = sheet.getLastRow()-1;
lastDate = Date(sheet.getRange("B"+(lastDateRow)+":B"+(lastDateRow).getValue());
The value you get from the sheet is already a date object (complete with year, date, hours etc), no further action should be necessary.
When you say ‘next date’ I guess you meant ‘next day’ ? If so, simply add 24*60*60*1000 to the date in milliseconds and you’ll get the next day 😉
In code it becomes something like this :
Logger result on a test :