How is the burndown calculated?

On each task card, in the upper-left hand corner is the task estimate for today. We record a single estimate per day. The red box around the number shows that an estimate for that item is due for today. And, currently, the day runs from noon till noon (before noon for yesterday and after noon for today). When a task card is moved to done an estimate of zero hours remaining is set. All the hours get tallied up for each day and are plotted on the burndown chart. If an estimate is not given for a day the system will use the previous estimate for the current day.