I have a hard enough time getting the fresh one down in the morning.
I store the old urine in the little pharmacy prescriiption bottles so I just use a little bottle as I need it. I happen to have some that are several weeks old and the smell is overwhelming but I dunno.
I usually use around 4 day old when it is really high like ammonia which I think works as a good astringent.
I am going to read those links . . . thank you for posting them.