Changes of hardness that correlate to time are more a matter of alloy mix and/or hardening by a quench or heat treat process. I'd not worry much about it frankly, certainly not for a time span of 30 days. Probably over the long haul degradation of lube is a more significant issue. My primary reference for this belief is having fired cast bullets that have been loaded for quite a long period, decades actually, without ill effect. Another small point, cast bullets purchased from them what makes them....what is the alloy? You don't know and neither do I. Shoot 'em today or tomorrow...or next year.

There's some info here on the age hardening issue:

http://www.lasc.us/HeatTreat.htm

scroll down almost to the page bottom for info related to age hardening of heat treated or quenched bullets.

I am..........disturbed.

Concerning the difference between man and the jackass: some observers hold that there isn't any. But this wrongs the jackass. -Twain