The Excel Forecast.Ets.Confint function calculates the confidence interval for a forecast value on a timeline.
The syntax of the function is:
Where the function arguments are:
target_date   
A date/time for which you want to predict a value. (Must be after the last date/time in the historical timeline). 

values    The array of historical known values for which you want to forecast the next point.  
timeline   
The independent array of dates/times, corresponding to each of the values. This must satisfy the following:


[confidence_level]   
An optional numeric value, indicating the confidence level for the calculated confidence interval. If supplied, this must be an integer between 0 and 1 (exclusive). If omitted, the [confidence_level] is set to the default value 0.95 (i.e. a 95% confidence level). 

[seasonality]   
An optional argument indicating the algorithm that should be used to detect seasonality in the data. If supplied, this must be a positive integer between 0 and 8784, meaning:


[data completion]   
An optional argument specifying how the algorithm should handle missing points in the timeline. If supplied, the [data completion] argument can have the value 0 or 1 meaning:


[aggregation]   
An optional argument specifying how the algorithm should aggregate values that have the same timestamp. If supplied, this can be any integer between 1 and 7 meaning:

Note: The Forecast.Ets.Confint function was first introduced in Excel 2016, so is not available in earlier versions of Excel. It is also not available in Excel 2016 for Mac.
A  B  

1  Month  Earnings 
2  Jan2015  879 
3  Feb2015  1259 
4  Mar2015  1230 
5  Apr2015  1471 
6  May2015  1638 
7  Jun2015  1371 
8  Jul2015  1562 
9  Aug2015  1526 
10  Sep2015  1125 
11  Oct2015  1340 
12  Nov2015  830 
13  Dec2015  726 
14  Jan2016  917 
15  Feb2016  1028 
16  Mar2016  1449 
17  Apr2016  1225 
18  May2016  1437 
19  Jun2016  1637 
20  Jul2016  1345 
21  Aug2016  1148 
22  Sep2016  1075 
23  Oct2016  928 
24  Nov2016  852 
25  Dec2016  752 
26  Jan2017  989 
27  Feb2017  1304 
28  Mar2017  1192 
29  Apr2017  1265 
30  May2017 
The above spreadsheet on the right shows a set of monthly earnings between Jan2015 and Apr2017. These values are plotted in the chart below:
The Forecast.Ets function has previously calculated the forecast earnings for May 2017 to be 1461.632054.
The Excel Forecast.Ets.Confint function can be used to calculate a confidence interval (with a confidence level of 95%) for this forecast earnings value, as follows:
This gives the result 202.1409609.
Therefore 95% of the time, the earnings value for May 2017 is expected to fall within 202.1409609 of the calculated value 1461.632054.
I.e. 95% of the time, the earnings value for May 2017 is expected to fall in the range:
For further details of the Excel Forecast.Ets.Confint function, see the Microsoft Office website
If you get an error from the Forecast.Ets.Confint function, this is likely to be one of the following:
#N/A    Occurs if the supplied values and timeline arrays have different lengths. 
#NUM!   
Occurs if either:

#VALUE!    Occurs if one or more of the supplied [confindence_level], [seasonality], [data completion] or [aggregation] arguments is nonnumeric. 