Eric’s Complete Guide To VT_DATE

I find software horology fascinating.

The other day, Raymond said "The OLE automation date format is a floating point value, counting days since midnight 30 December 1899. Hours and minutes are represented as fractional days."

That's correct, but actually it is a little bit weirder than that. I suspect that I may be the world's leading authority on bugs having to do with the OLEAUT date format, a dubious distinction at best.

Here are some interesting (well, interesting to me) facts about OLEAUT dates.

First of all, let's start with the obvious problem -- Midnight 30 December 1899 in what time zone? We never say. OLEAUT dates are always "local" which makes it very difficult to write code that uses OLEAUT dates -- any VB or VBScript program, for example -- which must deal with two things happening at the same time in different time zones.

Next, what about daylight savings time? How does one represent those days which due to springing forward or falling back have 23 hours or 25 hours? Again, those who use the OLEAUT date format need to pretend that these days do not exist.

Now let's get really weird. An OLEAUT date is, as Raymond noted, a double where the signed integer part is the number of days since 30 December 1899 and the fraction part is the amount of that day gone by. So what is 1.75? That's 6PM , 31 Dec 1899. What about -1.75? That's 6 PM 29 Dec 1899.

What about 0.75 and -0.75? Uh, those are zero and "minus zero" days from 30 December 1899, 6 PM . Those are the same time. This means that any program which must calculate the difference between two OLEAUT dates must say that -0.75 - 0.75 = 0.

The reason I know all this is because my first ever checkin as a full timer was a rewrite of the VBScript implementations of DateAdd and DateDiff, both of which used to be a mass of spaghetti code to handle all the special cases entailed by the discontinuities between -1.0 and 1.0. Incidentally, I now ask interview candidates to write me those algorithms during interviews! (Attention potential interview candidates reading this: writing a mass of spaghetti code on my whiteboard is a bad idea. Solving this problem by cases is a bad idea. There are better ways to solve this problem!)

Here's another bogus one: how about -1.99999999 and -2.0? Those are 0.00000001 apart in numbers but almost 48 hours apart in time! But OLEAUT dates are rounded to the nearest half second by the operating system, and you guessed it: any dates less than a quarter second before midnight before 30 Dec 1899 are sometimes "rounded" two days wrong. I wrote the code that converts OLEAUT dates to JScript dates, and it at least does correctly handle this case, though I had to jump through some hoops to do it.

Some more oddities: The range is enormous, and the precision varies greatly over the range! You can represent dates long before the creation of the Universe, though you lose precision as you go. The only valid dates for the date format are between 100 AD and 10000 AD, and since we have half-second granularity, we are wasting a whole lot of bits here.

And finally, why 30 December 1899? Why not, say, 31 December 1899, or 1 January 1900 as the zero day? Actually, it turns out that this is to work around a bug in Lotus 1-2-3! The details are lost in the mists of time, but apparently Lotus 1-2-3 used this date format but their devs forgot that 1900 was not a leap year. Microsoft fixed this bug by moving day one back one day.

Getting date code right is hard this is one of those areas where messy human requirements are hard to translate into crisp machine logic. There are huge localization problems for instance, in Japan it is legal to specify years as the fifth year of the reign of Emperor Hirohito . Thailand s calendar doesn t count from 1 AD. In Israel , the start and end days for daylight savings time are not standardized but rather are declared anew by the government after fierce debate every year.

These situations are fraught with peril for the unwary developer. I once drove our Microsoft support team in Israel to distraction by accidentally changing both Arabic and Hebrew locales to display dates right-to-left apparently Arabs really do read dates right-to-left but Israelis use left-to-right dates even when they are embedded in right-to-left Hebrew text.

You wouldn t think that something as simple as what day is it? could lead to so many problems, but the world is seldom as cut-and-dried as we software developers would like.

UPDATE: Some more history on this bizarre date format can be found in this article by Joel Spolsky.

Comments (11)
  1. Mike Dimmick says:

    It’s a hard problem.

    The Right Way (IMO) is to store and process all dates and times using UTC, then translate to the user’s locale when presenting data.

    I would have expected that -1.75 would be 0600 29 Dec 1899, not 1800. To do otherwise massively distorts people’s interpretations and expectations with regard to number lines.

    It’s strange that OLEAUT works like this when Windows also has two perfectly good ways of representing dates and times in SYSTEMTIME and FILETIME. But I suppose those didn’t exist in Windows 3.x when OLE 2.0 was first implemented.

  2. Merlin Beedell says:

    I could do with code samples on creating VT_DATE values in all the glorious ways that different versions of C++ allow. Or at least some pointers to useful resources.

  3. Eric Lippert says:

    A VT_DATE is fundamentally just a double-precision floating point, so "creating" one is not hard.

    If you’re asking how, for instance, to _translate_ strings into dates and vice versa, in various languages, the OLEAUT32.DLL API "VariantChangeTypeEx" is your friend.

  4. Tanveer Badar says:

    Actually, Joel Spolsky wrote some where in his blog about the bug in Lotus 1-2-3 that caused first day to be 30 Dec 1899, not  anything else.

  5. rolfhub says:

    @Tanveer Badar

    You probably mean this article:

    Quite an interesting and amusing read IMHO …

  6. Pierre B. says:

    You’ll get traffic today thanks to Raymond Chen! 🙂

    The problem of date manipulation does not look hard to me, and the solution is actually given in the specification: the integer days are signed, the fractional day is not, so do calculations by separating them in two values. -1.9999999 becomes -1 + abs(-0.9999999), rounding it is simply 1 + round(abs(-0.99999999)) = 0. Easy as pie!

    No one is suggesting it is hard once you realize that you have to do the transformation before the rounding. The problem with the format is that (1) it is easy to not realize that you need to do this transformation before any addition or subtraction, including rounding, and (2) double-precision floats are notorious for introducing small representation errors on trivial calculations; such an error in the wrong direction could in fact introduce an error of two days, not an fraction of second. — Eric

  7. mikeb says:

    Indeed once you realize what the specification is, the solution is simple, at least in principle – dealing with the vagaries of floating point rounding and discontinuities is always a pain – I’m glad I rarely have to deal with floating point.

    But I have to admit, I had to read Eric’s description a few times before I ‘got’ the spec – it’s quite counter intuitive (to me anyway). I haven’t yet looked at Raymond’s article or the true spec – maybe things are clearer there, but I suspect an awful lot of people get bit by assuming things that aren’t really in the spec. I wonder if I were given this interview question if I would have even been able to understand it well enough in that environment, or if I would have been stuck thinking, "WTF?".

    As an aside – I find this kind of ‘code archaeology’ interesting.

  8. bkohn says:

    I wouldn't want to work for someone who believes an interview question like that really tells you if you have a quality candidate.

    The purpose of the coding question is not to determine if you definitely have a high-quality candidate in your office; it is to determine if you definitely have a low-quality candidate in your office. When posing this question I have discovered that, for example, the candidate was unable to write a program that computes the absolute value of the difference between two perfectly ordinary doubles. That candidate did not get an offer; they were unlikely to do well at Microsoft. The purpose of the "design" portion of the question is also to eliminate poor candidates rather than to identify excellent candidates. Candidates that assume, for example, that everyone lives in the same time zone, or that there are no leap years, or that daylight savings time never happens, are unlikely to do well at Microsoft. — Eric

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