Magic numbers vs named constants

Question

When writing code, especially when dealing with dates and times, you have to work with a lot of specific numbers, eg: 60 seconds in a minute, 3600 = seconds in an hour.

Some people stick to using the raw values for many of these, whereas others put them into constants to increase readability.

eg:

$x = time() + 3600;
$y = time() + 86400;
$z = time() + 604800;

// vs

define('MINUTE', 60);
define('HOUR',   60 * MINUTE);   // 3600
define('DAY',    24 * HOUR);     // 86400
define('WEEK',    7 * DAY);      // 604800

$x = time() + HOUR;
$y = time() + DAY;
$z = time() + WEEK;

Of course, the second one is easier to read, but slightly OTT for some of the lower values, so where exactly do you draw the line? Personally, I see no problem with the readability of 86400 (in my head I automatically read that as "24 hours"), but would draw the line at the WEEK constant.

Answer 1

86400 is not ok, since you can easily mistype it as 84600, 88400, etc

A mistyped constant will be a compile error

Answer 2

One of my professors once told us not to put any magic numbers in our code except 1, -1, and 0. That's a little extreme, but it sticks in my mind and still guides me although I don't adhere to it completely.

In your example, I would prefer the symbolic names in all cases.

Answer 3

I'd go constants (or some cutesy derivative, like Rails' 15.minutes convention) pretty much everywhere. For me, it's about simplifying the "typing" of it all; if I see "10 * MINUTES" somewhere in a line, I know I'm dealing with time (or someone's up for an arse-kicking). If I see 10 * 60 or 600 it's entirely possible that I might not grok that we're dealing with time quite so easily.