Extract selected data based on time wave

Try using the interpolate dialog. Create the four waves first, then take select the 2nd time wave as destination coordinates.

Interpolate2/T=1/Y=shortdatawave/X=shorttimewave/I=3 longtimewave,longdatawave

Thanks a lot! That worked perfectly,  I would have never found interpolate2 as a solution, since in the help browser that function is described as a smoothing technique.  

Great that your problem has been resolved. Just to avoid confusion: You may have misread the possible option to use a 'smoothing spline' for interpolation as some kind of smoothing operation. But Interpolate2 does as the name says just an interpolation, i.e., it constructs new values between a discrete set of given values. In fact, with the option /T=1 (linear interpolation) as proposed by Tony there is no smoothing done at all.