Since DateTime already provide all required functions, you should be able to figure by yourself what to do. Fo example, it is very easy to call AddYears(60.0).
Then depending on the exact logic you need you can call AddMonths and/or AddDays and if you need to be at the beginning or end of a month... you create another DateTime with current year and month and 1 as the day.
If you need last day of a month, then you first compute first day of next month and then substract one day.
For example to get first day of a month, you do
DateTime date = something;
var firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
You can also copy information about local/UTC time if desired at that point using the appropriate constructor.
To get the last day of that month, you can do:
var lastDayOfMonth = firstDayOfMonth.AddMonth(1.0).AddDays(-1.0);
To be on the safe side, if you want last day of a month first get first day of that month. I'm not sure how .NET will handle
AddMonths(1)
if we are at a date that does not exist in next month. For example, if we add 1 month to last day of January.