For the purpose of vaccine spacing, what constitutes a month: 28 days (4 weeks), 30 days, or 31 days?

For intervals of 3 months or less, you should use 28 days (4 weeks) as a “month.” For intervals of 4 months or longer, you should consider a month a “calendar month”: the interval from one calendar date to the next a month later. This is a convention that was introduced on the childhood schedule in 2002 and discussed in the paper “Evaluation of Invalid Vaccine Doses” (Stokley S, Maurice E, Smith PJ, et al. American Journal of Preventive Medicine, 2004; 26[1]: 34–40).