.Net's is pretty good.

    var date = DateTime.UtcNow.AddDays(24).AddYears(-10);

In use:

   var billFor = 12; // months

   var firstDayOfMonth = DateTime.Today
       .AddDays(1 - DateTime.Today.Day);
    
   var billDates = Enumerable.Range(billFor)
       .Select(n => firstDayOfMonth.AddMonths(n));