If we were starting over, additions such as "1stMon" would be the way to go. However, given all those time zone compilers out in the field that don't understand "1stMon", and given that there's a way of representing the desired input that those compilers do understand, my sense would be to stick with the existing mechanism in the data files. --ado

...

And while we're on that subject, there are several lastSuns and a lastSat in Zone entries (which inherently refer to just one occurrence). Would it be better to replace those with the appropriate actual days?

How about avoiding >= -v- <= issues altogether, along with the occasional counting problems, by introducing synonyms like "1stMon", "2ndSun", etc. While not strictly necessary like their "last..." brethren, if clarity of expression is the goal then I proffer that they are just the ticket.