Last Update: 2/3/2015
The method used for the FFT assumes the time series is equally spaced. When some data points are absent, masked, or otherwise invalid, then the method may produce invalid results and the FFT dialog does not permit it. There are numerous methods for handling this problem and details of several methods are included in section 13.8 Spectral Analysis of Unevenly Spaced Data on pages 575-584 in the reference below. The simplest Origin technique is to use Analysis : Mathematics : Interpolate to generate evenly spaced data.
Numerical Recipes in C: The Art of Scientific Computing by William H. Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vetterling . Publisher: Cambridge University Press 2 edition (October 30, 1992)
Keywords:sampling, interval, periodic, uneven, missing