What is the greatest number of days that can occur after the first of one month and before the first of the next month? 
Overseas Competition->Knowledge Point->Word Problem Modules->Periodic Problems->Periodic Problems of Time
A month has at most $$31$$ days, Hence, the greatest number of days in that month, after the first, is $$31 -1=30$$. Then, the next day is another $$1\text{st}$$. 