Monthly inspection does not mean every 30 days. It does not either mean any day of the month.

The difference between "Monthly" and "every 30 days" is that the word "monthly" is little flexible while "every 30 days" is not.

So if there is a requirement to inspect something at least every 30 days, we cannot go beyond 30 days. The interval between two inspections needs to be less than 30 days.

And if the requirement is for monthly inspection, the time interval between two inspections can be a little more than 30 days but only in certain circumstances where it was not possible to inspect the equipment on and near to the 30th day.

This means that while the meaning of the word "monthly" is not exactly "30 days" but at the same time it does not mean that we inspect something on the 01st of March and then on 30th of April. As far as possible, it should be close to 30 days but we are allowed to go a little over 30 days.

I am not sure if there is any document to support this but this explanation is purely based on the logic.