Ahh Hello there,
If I may add to this confusion: Is that Kate does not want to interpolate into the past and instead would like to shift all the time series so that they align to a forward point in time. In that case would the option -tzero or -slice (which allow you so set the offset to align time series to) solve the problem?
Having suggested that, what is wrong with going backwards? The default of shifting to align with the middle offset was chosen to minimize the net amount of shift any one time series is subjected to.
I have to run now or I will miss my bus.
cheers,
-z