LocalDateTime, UniversalDateTime... how to compare

  • Hi,

    I need to get the current time and some other time and compare them. Both are possibly in different time zones, so my guess is that using UniversalDateTime and converting to LocalDateTime is the way to go here. I need to find out the difference between currentDate and otherDate in days.

    I could not find an example for this.

    This is the code so far:

    // Get current date
    maxon::LocalDateTime currentDate(maxon::LocalDateTime::GetNow());
    // Format string for date parsing
    const Char* dateFormatString = "%Y%m%d";
    // Some other date
    String otherDateString = String("20200801"); // For testing, use August 1st 2020
    const maxon::LocalDateTime otherDate = maxon::LocalDateTime::FromString(otherDateString, dateFormatString) iferr_ignore();
    // How many days between the dates?
    maxon::UniversalDateTime difference = otherDate.ConvertToUniversalDateTime() - currentDate.ConvertToUniversalDateTime(); // Obviously, not correct

    Thanks in advance & cheers,

    EDIT: Originally, I had posted two questions. The solution to the first one came to me right after posting, so I removed the question.

  • Inspired by a posting on Stackoverflow and a page in the SDK docs, I tried it like this:

    std::tm stdCurrentDate = maxon::ConvertLocalDateTimeToTM(currentDate);
    std::tm stdOtherDate = maxon::ConvertLocalDateTimeToTM(otherDate);
    Int32 daysDifference = (Int32)(std::difftime(std::mktime(&stdOtherDate), std::mktime(&stdCurrentDate)) / (60 * 60 * 24));

    But it does not compile, because there is no ConvertLocalDateTimeToTM() in the maxon namespace.

    I learned about that function on this documentation page: https://developers.maxon.net/docs/Cinema4DCPPSDK/html/page_maxonapi_maxon_convertlocaldatetimetofromtm.html, which header do I have to include to make it work? It does not seem to be maxon/ansi_type_conversions.h, as that gives me several errors when included.

  • By the way, the above posted code also throws a rather strange error:

    No member named 'FORBIDDENI_CALL_USE_mktime_r' in namespace 'std'
    Is it possible that it's supposed to be FORBIDDEN_CALL_USE_mktime_r, without the "I" ?

    Also, the documentation for that macro is a bit confusing (ansi_type_conversions.h, line 122):
    // The following functions should be avoided. On Windows and OSX these functions are thread-safe, but not on Windows.
    So, are they thread safe on Windows, or are they not?

  • Hm,

    I might be overlooking something here, as I just did look at the the docs, but are the arithmetic operators for UniversalDateTime not only defined for TimeValue as being the other operand (Signature for the difference operator), while you are trying to invoke it on another UniversalDateTime?

    Have you considered to just subtract the Unix epoch format representation of your values (via UniversalDateTime::GetUnixTimestamp()) and then convert that result into the desired output format (e.g. days) by simple division?

    edit: eh, I did overlook the "obviously not correct" comment, due to being on an iPad, but the rest should still apply.


  • That sounds feasible, I'll try that. Thanks!

  • However, I would still like to know how I can (or why I can't) use ConvertLocalDateTimeToTM(), as it is listed in the SDK docs.

  • I tried like this:

    // Format string for date parsing
    // Current date
    // Get expiry date
    expirationDateString = String("20191025");
    maxon::LocalDateTime expirationDate = maxon::LocalDateTime::FromString(expirationDateString, dateFormatString) iferr_ignore();
    expirationDate._hour = 0;
    expirationDate._minute = 0;
    expirationDate._second = 0;
    expirationDate._daylightSavingTime = maxon::DST::AUTOMATIC;
    // Convert local to universal datetime
    maxon::UniversalDateTime universalCurrentDate = currentDate.ConvertToUniversalDateTime();
    maxon::UniversalDateTime universalExpirationDate = expirationDate.ConvertToUniversalDateTime();
    // Convert universal datetime to Unix timestamp
    UInt64 currentDateStamp = universalCurrentDate.GetUnixTimestamp();
    UInt64 expirationDateStamp = universalExpirationDate.GetUnixTimestamp();
    // Subtract timestamps
    UInt64 remainingDaysStamp = expirationDateStamp - currentDateStamp;
    // Convert seconds to days
    Int64 daysDifference = remainingDaysStamp / (60 * 60 * 24);

    But it gives me an incredibly high value for daysDifference, even though the dates are just 4 days apart.

    Printing the values to the console, these are the surprising results:

    Current date LOCAL: 2019-10-29 10:51:20 (that is correct!)
    Expiration date LOCAL: 2019-10-25 00:00:00 (that is what I expected, too)
    Current date UNIVERSAL: 2019-10-29 09:51:20 (that is also correct)
    Expiration date UNIVERSAL: 2019-10-24 22:00:00 (why 22:00 ??)
    Current date UNIX: 1572342680
    Expiration date UNIX: 1571954400
    Days difference: 213503982334596 (why??)

    How cumbersome can it be to just subtract two dates from each other?
    If I just knew how to use ConvertLocalDateTimeToTM(), the problem would've been long solved.

    And why is daysDifference so large? If I calculate it myself with a pocket calculator, I get this:

    (1571954400 - 1572342680) / (60 * 60 * 24) = -4,4939814815

    And that is exactly why I would expect: About four and a half days.

  • Hi,

    hm, I am not much into the finer details of C++ (in this case how C++ handles implicit casts like this), but could it be, that the reason is, that the Unix time stamps returned by the date time objects are unsigned integers (which strikes me as an odd choice anyways, since they are also 64 bit) and you implicitly cast them into a signed integer carrying over an integer overflow? So maybe try casting them into signed integers before or just sort out min/max conditions before.


  • Now that you mention it, I just noticed it, too.

    I guess that was my daily dose of shame.... :-/

    remainingDaysStamp needs to be Int64, not UInt64.

    Thank you so much, that brought me back on track!


  • Hi,

    jeah, oops, I somehow red remainingDaysStamp as being Int64, mixing it up with daysDifference below. This is a straight up integer overflow. But I am not quite sure if changing remainingDaysStamp will be enough, you might have to convert your operand values to signed integers. At least in older versions of C# the compiler behaved that way (trying to implicitly cast int my_int = smaller_uint - bigger_uint; gave you the overflow value).


  • I got it working now. Thank you! :-)

  • Hi Frank, looks like I came too late here :)

    Thanks @zipit and @fwilleke80 for running the conversation.
    Couple of notes:

    • do all the maths by using the UniversalDateTime as pointed in our documentation: I don't see any major advance to use the LocalDateTime
    • use the maxon::UniversalDateTime::FromValues() to convert from year, month, day, hour, min, sec to maxon::UniversalDateTime representation
    • in case also hours, mins and secs difference gets interesting you can easily extend the code to consider those differences
    • consider that otherDate shouldn't be necessarily bigger than currentDate
    • make proper use of error handling.

    Below my suggested version

    	// get the current time in UniversalDateTime
    	const maxon::UniversalDateTime currentUDate(maxon::UniversalDateTime::GetNow());
    	// get the other time in UniversalDateTime
    	const maxon::UniversalDateTime otherUDate = maxon::UniversalDateTime::FromValues(2019, 10, 01, 00, 00, 00) iferr_return;
    	// compute the timestamp difference between the twos
    	UInt64 utDifference = 0;
    	if (currentUDate > otherUDate)
    		utDifference = currentUDate.GetUnixTimestamp() - otherUDate.GetUnixTimestamp();
    		utDifference = otherUDate.GetUnixTimestamp() - currentUDate.GetUnixTimestamp();
    	// allocate some conversion factors
    	const Float toMin = 60;
    	const Float toHour = 60 * toMin;
    	const Float toDay = 24.0 * toHour;
    	// compute the respective temporal differences
    	const UInt days = UInt(utDifference / toDay);
    	const UInt hours = UInt((utDifference - days * toDay) / toHour);
    	const UInt mins = UInt((utDifference - days * toDay - hours * toHour) / toMin);
    	const UInt secs = UInt(utDifference - days * toDay - hours * toHour - mins * toMin);
    	// just print
    	if (currentUDate > otherUDate)
    		DiagnosticOutput("Difference between @ and @ is \n\t@ days\n\t@ hours\n\t@ mins\n\t@ secs", currentUDate, otherUDate, days, hours, mins, secs);
    		DiagnosticOutput("Difference between @ and @ is \n\t@ days\n\t@ hours\n\t@ mins\n\t@ secs", otherUDate, currentUDate, days, hours, mins, secs);
    	return maxon::OK;

    Cheers, R

  • Thank you, Ricardo! :-)