fix nim-lang/RFCs#211: var a: DateTime works

lib/pure/times.nim

@@ -260,6 +260,7 @@ type
   Month* = enum ## Represents a month. Note that the enum starts at ``1``,
                 ## so ``ord(month)`` will give the month number in the
                 ## range ``1..12``.
+    # mInvalid = (0, "Invalid") # intentionally left out so `items` works
     mJan = (1, "January")
     mFeb = "February"
     mMar = "March"
@@ -296,7 +297,8 @@ when defined(nimHasStyleChecks):
   {.pop.}
 
 type
-  MonthdayRange* = range[1..31]
+  MonthdayRange* = range[0..31]
+    ## 0 represents an invalid month
   HourRange* = range[0..23]
   MinuteRange* = range[0..59]
   SecondRange* = range[0..60]

tests/stdlib/ttimes.nim

@@ -614,6 +614,16 @@ suite "ttimes":
       doAssert x + between(x, y) == y
       doAssert between(x, y) == 1.months + 1.weeks
 
+  test "default DateTime": # https://github.com/nim-lang/RFCs/issues/211
+    var a: DateTime
+    doAssert a == DateTime.default
+    doAssert ($a).len > 0 # no crash
+    doAssert a.month.Month.ord == 0
+    doAssert a.month.Month == cast[Month](0)
+    var num = 0
+    for ai in Month: num.inc
+    doAssert num == 12
+
   test "inX procs":
     doAssert initDuration(seconds = 1).inSeconds == 1
     doAssert initDuration(seconds = -1).inSeconds == -1