BarHistory-TradingDaysBetweenDates-unexpected-output

Notice in the code below, I call the BarHistory.TradingDaysBetweenDates(...) property twice with its date parameters switched. I one case I get "trading days" between dates. In the other case, I get "calendar days" between dates. Is this inconsistent behavior by design or a bug? I think this behavior is confusing. I do want the negative value, but I want it to represent "trading days", not "calendar days".

CODE:
using WealthLab.Backtest;
using WealthLab.Core;
using WealthLab.Indicators;
using System.Collections.Generic;
using WealthLab.Data; //* Fundamental event data here *//
using WealthLab.Fundamental;

namespace WealthScript2 
{
    public class MyStrategy : UserStrategyBase
    {
        public override void Initialize(BarHistory bars)
        {
         List<EventDataPoint> earnings = bars.GetEventDataPoints("Earnings"); //most recent earnings
         if (earnings.Count != 0)
         {
            EventDataPoint fiEPSmrq = earnings[earnings.Count-1];
            int tradingDaysUntilEarnings = bars.TradingDaysBetweenDates(bars.DateTimes[bars.Count-1], fiEPSmrq.Date); //in past, so negative
            WriteToDebugLog(tradingDaysUntilEarnings.ToString("0=tradingDaysUntilEarnings"));
            tradingDaysUntilEarnings = bars.TradingDaysBetweenDates(fiEPSmrq.Date, bars.DateTimes[bars.Count-1]); //in past, so negative
            WriteToDebugLog(tradingDaysUntilEarnings.ToString("0=tradingDaysUntilEarnings"));
         }
      }

      public override void Execute(BarHistory bars, int idx) { }
    }
}