Fixed completely broken day_since_earliest

Well this explains the "NA" we're seeing. I don't know how I missed this. :(
These sentinels should really be null or another type (i.e. implement an efficient option as a cell with a flag). Having an invalid date is waiting for an accident. I know we have talked about this before, but I am beyond uncomfortable with it, it undermines usages of the Date type, and  I can see bugs where we output or silently use this sentinel date as an actual value.