How to pad real-valued sequences
How you pad it (and even whether you do so) would depend on what you expect of the data. This imposes boundary conditions on the data which will induce artifacts in any transform you make. How bad this effect depends on how well geared your data is to accepting a particular padding method.

Padding methods include zero padding or a periodic bound.

Padding doesn't have to be done in the time domain. Eg interpolating in the frequency domain and back transforming allows you to extrapolate.

If your analytics has a finite history (eg FIR filters) then you can isolate time regions where padding is unnecessary and draw comparisons therefrom.