As far as your main point, I think your example/argument of dates never evaluating to false while times sometimes do is valid. On the other hand, someone might argue that something that can have a 0 value (such as a time) should be false when it's 0. But a date cannot really have a 0 value (since, as you implied, there is no 0th day of year 0 [disregarding whether there can actually be a year 0] and no 0th month - so you can never have a date [year/month/day] that evaluates to 0) and thus it's appropriate for a date to never evaluate to false. (This is not my argument, but one I think some people would make.)
To address that last argument, let me bring in the Baby object from my earlier post. You might have Baby == 0 (AKA baby.age == 0) and you might think of that as false. But why? What does false mean? It exists [otherwise it would be null/void and not evaluate to anything :-)] - doesn't existence imply truth?
I suppose I'm actually generalizing the argument - partly because some people will object to your argument for the reason I gave. I suggest that for a system that will be around for years and will be modified and extended, Boolean checks should be explicit (i.e., there should be no auto-conversion of a type into Boolean) - Instead of if (not currenttime), the code should read, for example: if (currenttime.is_midnight). IMO, this is clearer and much less error-prone. It's less likely that someone changing the code will make a mistake because the intent of the if ... clause wasn't clear to him. (I suppose you could say this is a long-winded way of saying what someone else suggested earlier: Make the code readable.)
I think part of the problem is the dual nature of languages such as perl and python (and, to some extent, ruby) - One purpose of these languages is to allow writing informal scripts (usually short and quick) that take care of an immediate need. The other is to develop applications, many of which will be quite large and will be around for a long time, and will be used by many people. For the first purpose it perhaps makes sense to allow, e.g., 0 => false; but for major projects I think it might not be a good idea.
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