public static DateTime GetLastDayOfMonth(DateTime? date = null)
{
    DateTime currentDate = date ?? DateTime.Now;
    var offsetDate = new DateTime(currentDate.Year, currentDate.AddMonths(1).Month, 1).AddSeconds(-1);
    // when returning 31 Dec the previous line would reduce the year by 1 so add it back
    return (offsetDate.Year < currentDate.Year) ? offsetDate.AddYears(1) : offsetDate;
}