Add comments on error caused by time approximations.

Some are corrected, others are TODO.

apachedex/__init__.py

@@ -813,6 +813,7 @@ period_parser = {
     # Longest month: 31 days
     timedelta(31),
     lambda x: x,
+    # Error margin without correction: 3/31 = 10%
     lambda x: 31. / calendar.monthrange(x.year, x.month)[1],
   ),
   'quarter': (
@@ -824,6 +825,7 @@ period_parser = {
     '%Y/%m/%d',
     timedelta(7),
     _roundWeek,
+    # Error margin without correction: (366 % 7 = 2) 2/7 = 29%
     _getWeekCoefficient,
   ),
   'month': (
@@ -834,7 +836,8 @@ period_parser = {
     # Longest day: 24 hours + 1h DST (never more ?)
     timedelta(seconds=3600 * 25),
     lambda x: x,
-    lambda x: 1, # XXX: take DST into account (1/24th) ?
+    # Error margin without correction: (DST) 1/24 = 4%
+    lambda x: 1,
   ),
   'week': ( # XXX: should be "7 days", but a single word is more convenient
     _as6HourString,
@@ -843,6 +846,7 @@ period_parser = {
     '%Y/%m/%d %H',
     timedelta(seconds=3600 * 6),
     _round6Hour,
+    # Error margin without correction: (DST) 1/6 = 17%
     lambda x: 1,
   ),
   'day': (
@@ -853,6 +857,7 @@ period_parser = {
     # Longest hour: 60 * 60 seconds + 1 leap second.
     timedelta(seconds=3601),
     lambda x: x,
+    # Error margin without correction: (leap) 1/3600 = .03%
     lambda x: 1,
   ),
 }