Calendars and Units

In our application, we use both a DailyCalendar and an HourlyCalendar. So we have the standard SI_Time_Duration quantity with second as a base unit and conversions to hour and day.

For the daily calendar, I want to model a period of time. So I have two element parameters EP_FirstDay and EP_LastDay with range DailyCalendar and a parameter P_LengthOfPeriod. As the length of the period is specified in days, I set the unit of it to [day].

However, if I set EP_FirstDay to the 1st of January 2020 and P_LengthOfPeriod to 1 [day], then EP_LastDay := EP_FirstDay + P_LengthOfPeriod results in 22nd of July in the year 2256 instead of the 2nd of January (which was quite surprising to me).

I understand, that P_LengthOfPeriod is converted to its base unit [s], so the above calculation actually adds 86400 days to EP_FirstDay. Of course, I could just remove the unit from P_LengthOfPeriod, but I would like to keep it, if possible.

Is there a better way of modeling these kind of situations?

Procedure PR_Example {
 Body: {
 EP_FirstDay := '2020-01-01';
 P_LengthOfPeriod := 1 [day];

 !Results in EP_LastDay = '2256-07-22' instead of '2020-01-02'
 EP_LastDay := EP_FirstDay + P_LengthOfPeriod;
 }
}


Quantity SI_Time_Duration {
 BaseUnit: s;
 Conversions: {
 hour->s : #-># * 3600,
 day ->s : #-># * 86400
 }
 Comment: "Expresses the value for the duration of periods.";
}


Calendar DailyCalendar {
 Unit: day;
 BeginDate: "2020-01-01";
 EndDate: "4000-12-31";
 TimeslotFormat: "%c%y-%m-%d";
}


ElementParameter EP_FirstDay {
 Range: DailyCalendar;
}


Parameter P_LengthOfPeriod {
 Range: integer;
 Unit: day;
}


ElementParameter EP_LastDay {
 Range: DailyCalendar;
}