In sheet Hidden (or something like this, I don't have Excel at home) in file, updated by me, is a table where in second column are calculated month end dates for whole months range. Look there for formula!

Generally, a last day of month equals with 0th day of next month.

Btw, in my example I based all calculations on daily depreciation - so all calculations for any time interval are uniform. According your last post, you want depreciation for all full months to be same, and for non-full months based on days in period. There will be several problems with it.
1. You have to apply several different calculation rules;
2. You have to decide, do you count non-full start and end months as a single full month, or as separate non-full months;
3. In case you count non-full start and end months as single full month, you have to decide, which rules do apply, when the period starts and ends in February, and either start or end month belongs to leap year;
4. And add cases, where the period is several years (and not full years).

There is a reason why banks and leasing firms prefer to start all leasing periods at 1st of start month and end at last of end month!