TagVal('TAG', FINDGE('TAG', '*', '*-365d', 0)) gives last good value. I have given time range for 1 year and value 0 based on the tag. If your tag has negative values then define the limit accordingly in FINDGE.
Please make sure starttime is *
Hope this helps.
So aside for giving an arbitrary end time there is no other way? I guess i'll have to make do. Also, thanks for this, i didn't know you could reverse the beg and end times!