Go to the Add functions tab in the Time function definition dialog.
Select a function from the list of available functions, and then click → to add it to the Function 1 box.
Enter a coefficient value for the first time function in the Factor box.
Repeat steps 2 and 3 for the second function.
Enter a name for the function in the Name box.
Click Summation. The resulting function is added to the list of defined functions.
Note: The value of the resultant function is interpolated at the points with common abscissae. Each of the defined functions can be multiplied by a user-defined coefficient.