You cannot hold this as datetime value, which can deal only with full seconds. Your logger seems to read 8 sets of values every second and in this case you would still need another column to keep the fraction of the second. And this is pretty much what you have right now.
But considering the amount of data (72k lines) I am not really sure if Calc is the right tool for the task unless you reduce the amount of data by e.g using averages over several seconds or so. You most likely want to visualize it somehow and it is impossible to get a diagram from a set of 72k values, Calc will freeze during this (it might eventually finish it, but then you probably will need to adjust your diagram).
So, considering all this, you are better off with python scripts and pyplot (or whatever you fancy) if you need visualization. Google will be your guide. Or if you ask nicely I can donate my scripts which I wrote two years ago for slightly similar purpose (temperature and humidity logging over almost a year).