|The Universe of Discourse|
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Fri, 10 Jan 2014
The DateTime suite is an impressive tour de force, but I hate its interface. The methods it provides are usually not the ones you want, and the things it makes easy are often things that are not useful.
The most obvious example is that it has too many mutators. I believe
that date-time values are a kind of number, and should be treated like
numbers. In particular they should be immutable. Rik Signes has
a hair-raising story
about an accidental mutation that caused a hard to diagnose bug,
But the most severe example, the one that drives me into a rage, is
For example, suppose you would like to know how much time elapses between 1969-04-02 02:38:17 EST and 2013-12-25 21:00:00 EST. You can set up the two DateTime objects for the time, and subtract them using the overloaded minus operator:
Internally this invokes
You might want to know how long this interval is, so you can compare
it to similar intervals. So you might want to know how many seconds
this is. It happens that the two times are exactly 1,411,669,328
seconds apart, but there's no way to get the
It seems like there are methods that will get you the actual
elapsed time in seconds, but none of them will do it. For example,
And indeed, no method can tell you how long the duration really is, because the subtraction has thrown away all the information about how long the days and months and years were—days, months and years vary in length—so it simply doesn't know how much time this object actually represents.
Similarly if you want to know how many days there are between the two dates, the DateTime::Duration object won't tell you because it can't tell you. If you had the elapsed seconds difference, you could convert it to the correct number of days simply by dividing by 86400 and rounding off. This works because, even though days vary in length, they don't vary by much, and the variations cancel out over the course of a year. If you do this you find that the elapsed number of days is approximately 16338.7653, which rounds off to 16338 or 16339 depending on how you want to treat the 18-hour time-of-day difference. This result is not quite exact, but the error is on the order of 0.000002%. So the elapsed seconds are useful, and you can compute other useful values with them, and get useful answers. In contrast, DateTime::Duration's answer of "536 months and 23 days" is completely useless because months vary in length by nearly 10% and DateTime has thrown away the information about how long the months were. The best you can do to guess the number of days from this is to multiply the 536 months by 30.4375, which is the average number of days in a month, and add 23. This is clumsy, and gets you 16337.5 days—which is close, but wrong.
To get what I consider a useful answer out of the DateTime objects you must not use the overloaded subtraction operator; instead you must do this:
What's DateTime::Moonpig for?
DateTime::Moonpig introduces five main changes to the interface of DateTime:
Under the covers, it is all just DateTime objects, which seem to do
what one needs. Other than the mutators, all the many DateTime
methods work just the same; you are even free to use
When I first started this module, I thought it was likely to be a failed experiment. I expected that the Moonpig::DateTime objects would break once in a while, or that some operation on them would return a DateTime instead of a Moonpig::DateTime, which would cause some later method call to fail. But to my surprise, it worked well. It has been in regular use in Moonpig for several years.
I recently split it out of Moonpig, and released it to CPAN. I will be interested to find out if it works well in other contexts. I am worried that disabling the mutators has left a gap in functionality that needs to be filled by something else. I will be interested to hear reports from people who try.