public IEnumerable<(DateTime start, DateTime end)> GetDatePeriods(DateTime start, DateTime end)
{
    if(start.Day > 1) 
      throw new ArgumentException(nameof(start), "Нужно передавать 1 число в начале периода.");
    while(start < end)
    {
        var next = start.AddMonth(1);
        yield return (start, next.AddSecond(-1));
        start = next;
    }
}

static void Main(string[] args)
        {
            //DateTime dnow = DateTime.Now;
            DateTime dnow = new DateTime(2019, 02, 28);
            Console.WriteLine(First(dnow));
            Console.WriteLine(Last(dnow));

            Console.ReadLine();

            
        }
        public static DateTime First(DateTime dt)
        {
            return new DateTime(dt.Year, dt.Month, 1);
        }

        public static  DateTime Last(DateTime dt)
        {
            return First(dt.AddMonths(1)).AddTicks(-1);
        }

where myDate >= startDate and myDate < endDate

where myDate between startDate and endDate

where myDate>= startDate and myDate < endDate