PC 49.09(a) says that there is a 30 day minimum jail sentence for DWI 2nd. Under CCP 42.12 - 13(a)(1) there is minumum jail time as a term of community supervision (3 days on DWI 2nd, 5 days if the two offenses were withing 5 years of each other).

My reading of these two statutes is that if a defendant is placed on probation for DWI 2nd, he must receive a jail sentence of at least 30 days, which can be probated. But if it is probated, then the Defendant must serve a minimum of either 3 or 5 days in jail as a term of probation (with credit for time already served).

Is this correct? Am I missing anything here?

I think you've got it right, except that I don't think the defendant gets any credit toward any time to be served as a condition of probation. And, at least in our county, he also won't receive any "good time" credit. It's day-for-day.