Comparing times and dates in Ruby

In one of the Rails projects I’m involved with, we do most of the local development against SQLite and then deploy against Oracle. This is a bit annoying for many reasons, but by far the largest cause of trouble is the handling of dates. I haven’t exactly figured out the rules, but for some reason sometimes Oracle returns DateTime in situations where SQLite returns a Date. This usually causes quite subtle problems that have effects in other parts of the application. This brings me to the small piece of advice I wanted to talk about in this column. Always make sure that you know if you are working with a Date, a Time or a DateTime, since these all have slightly different behavior, especially when it comes to comparisons.

The rule is quite simple. If you think you can have a Time object, make sure to turn it into a DateTime object before trying to compare it to a Date object. What happens otherwise? Unfunny things: < #ArgumentError: comparison of Date with Time failed > #true == #false == #nil <=> #nil != #true

The first time I saw some of these results, I was a bit confused. Especially the last three. But they do make a twisted kind of sense. Namely, it’s OK for the <=> operator to return nil if it can’t do a comparison between two objects. And the != in Ruby is hardcoded to return the inverse of the value returned from ==, and the since nil is a falsey value, the inverse of that becomes true.

What I wanted to mention with these things is that you should always make sure you don’t have the Date on the left hand side of a comparison. Or if you want to do a comparison, explicitly call to_date to coerce them. Finally, if you want to do date and time comparisons, I find the best behavior usually comes from coercing both sides with to_datetime before doing the comparison.

Joda Time

I spent a few hours this weekend converting RubyTime in JRuby to use Joda Time instead of Calendar or Date. That was a very nice experience actually. I’m incredibly impressed by Joda, and overall I think it was worth adding a new dependency to JRuby for this. The API is very nice, and immutability in these classes make things so much easier.

There were a few things I got a bit annoyed at though. First, that Joda is ISO 8601 compliant is a really good thing, but I missed the functionality to tune a few things. Stuff like saying which weekday a week should start on, for the calculation of current week would be very nice. As it is right now, that functionality has to use Calendar. It might be in Joda, but I couldn’t find it.

The other thing I had a problem with – and this actually made me a bit annoyed – was how Joda handles GMT and UTC. Now, it says clearly in the documentation that Joda works with the UTC concept, and that GMT is not exactly the same thing. So why is it this code passes (if assertNotEquals is assumed):

    public void testJodaStrangeNess() {
assertEquals(DateTimeZone.UTC, DateTimeZone.forID("UTC"));
assertEquals(DateTimeZone.UTC, DateTimeZone.forID("GMT"));
assertEquals(DateTimeZone.UTC, DateTimeZone.forOffsetHours(0));
assertNotEquals(DateTimeZone.UTC, DateTimeZone.forID("Etc/GMT"));
assertNotEquals(DateTimeZone.forID("GMT"), DateTimeZone.forID("Etc/GMT"));

Yeah, you’re reading it right – UTC and GMT is the same time zone. +00:00 is the same as UTC too. But Etc/GMT is not the same as UTC or GMT or +00:00. Isn’t that a bit strange?