simplify single date parsing

[alexduryee]

For single dates (e.g. July 4, 1776), there are currently a number of forms that they can take (e.g. 1776 July 4, 1776 4 July, etc.). Notably, match_replace parser 80 compares like forms to like (e.g. July 4, 1776 - July 5, 1776) but not unlike (not e.g. July 4, 1776 - 5 July 1776). It's not inconceivable that this could happen, so the parser should be able to field mixed-form dates in parser 80.