You need to provide a solid baseline (history) in order for your Excel forecast functions to work accurately in Excel sales forecasting. This chart shows you some ways to arrange data for your baseline:

The Issue How to Deal with the Issue
Order Put your historical data in chronological order, earliest to latest.
Time periods Use time periods of approximately equal length: all weeks, all months, all quarters, or all years.
Same location in time If you're sampling, then sample from the same place. Don't take January 1, February 15, March 21. Instead, use January 1, February 1, March 1, and so on.
Missing data Missing data is not allowed. If you have every month except, say, June, find out what June's sales were. If you can't, get the best estimate possible — or start your forecasting with July.