That can't be done in hai, but I thoughtof another way to do it: create a flag named like YesterdayWasEven, then on the 1st of every month set it to 1 and everyday at, say, 00.10 switch its value (there's an action for that in the HAI conteoller, or a simple IF on THEN off and IF off THEN on would do.
Prior to that at, say, 00.05, the 1st of every month the central would check if the last of the previous month was odd or even and set the main even/odd variable accordingly.
Hope it was clear.
Maybe in this particular case doing it in automatic is not shorter than manually set every month, but I can think of cases where this may me in handy. I'm thinking of irrigation that must start every other day and so can't be setup using weekdays (that are odd) or things like that, who knows?
Also I would consider setting a flag to 31 and decrease it everyday: if it's 0 (off) at the end of the month then it had 31 days, else it had 30 (or 28, or 29 the leap year must be taken in account separately) and the flag is considered 'on' (you can't directly check the value unfortunately).
It' getting complicated, maybe if he tells us what must be done we can put down some code.