There are two types of out of date markings....Best Before and Use By.
Best Before means the product will taste better if used before the date printed on it. Usually on dried and processed foods....think pasta, rice, biscuits, even beer...... ect
Use By should be treated as more of a warning as it is usually associated with fresh foods such as meats, cheese, milk ect, that could be damaging to our health if consumed after a certain date as they start to rot......
Personally I would eat the bar as its processed, I have always allowed up to 6 months after any best before date and never had a problem with any products.....think about it there is even a best before date on bottled water! How possibly could water go 'off' when in a sealed sterilised container?
I hope this helps. XX