Converting weird date strings to usable values?

I'm downloading time-series data from a web portal. The rows are dates in the format

d_YYYYMMDD

Data -> text-to-columns takes care of the concatenation, separating the "d_" from the value. Problem is excel seems to hate the resulting YYYYMMDD numbers and no matter which format I convert it to it spits out an infinite string of "#####"

Any tips / workarounds? I don't care what format the date ends up in, as long as excel understands it.