Can't figure out how to get the Gantt chart to update when dates are changed on (Product Launch Checklist)

Hello,

Please help me as soon as possible.  I'm a bit new to Excel.

I have the Product Launch Checklist sheet attached.  

The dates are currently for the year 2011.  I need to modify the dates to dates in year 2018.  When I modify the dates, the Gantt chart no longer appears.  I can't seem to figure out how to fix this.  Please help!

Thanks,

Joseph Thomas