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 :
function xxx(){
var sheet = SpreadsheetApp.getActiveSheet()
var lastDateRow = sheet.getLastRow();
var lastDate = sheet.getRange(lastDateRow,2).getValue();
Logger.log(lastDate)
var nextDay = new Date(lastDate.getTime()+24*60*60*1000);
Logger.log(nextDay)
}
Logger result on a test :
Tue Aug 21 15:00:00 PDT 2012
Wed Aug 22 15:00:00 PDT 2012
Actualy this is what I ended up with after looking up js nextdate fnction here on stackowerflow.
var lastDateRow = sheet.getLastRow();
var lastDate = sheet.getRange(lastDateRow,2).getValue();
lastDate = getTomorrow(lastDate,1);
function getTomorrow(d,offset) {
if (!offset) { offset = 1 }
return new Date(new Date(d.getTime()).setDate(d.getDate() + offset));
}