fix https://github.com/nim-lang/RFCs/issues/211: var a: DateTime compiles and is usable (#14002) [backport:1.2]

* fix https://github.com/nim-lang/RFCs/issues/211: `var a: DateTime` works
* assertValidDate checks for sentinel month

(cherry picked from commit e3919b658f)

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 day of the month
   HourRange* = range[0..23]
   MinuteRange* = range[0..59]
   SecondRange* = range[0..60]
@@ -670,7 +672,7 @@ proc getDaysInYear*(year: int): int =
 
 proc assertValidDate(monthday: MonthdayRange, month: Month, year: int)
     {.inline.} =
-  assert monthday <= getDaysInMonth(month, year),
+  assert monthday > 0 and monthday <= getDaysInMonth(month, year),
     $year & "-" & intToStr(ord(month), 2) & "-" & $monthday &
       " is not a valid date"
 
@@ -1576,9 +1578,14 @@ proc `<=`*(a, b: DateTime): bool =
   ## Returns true if ``a`` happened before or at the same time as ``b``.
   return a.toTime <= b.toTime
 
+proc isDefault[T](a: T): bool =
+  system.`==`(a, default(T))
+
 proc `==`*(a, b: DateTime): bool =
   ## Returns true if ``a`` and ``b`` represent the same point in time.
-  return a.toTime == b.toTime
+  if a.isDefault: b.isDefault
+  elif b.isDefault: false
+  else: a.toTime == b.toTime
 
 proc isStaticInterval(interval: TimeInterval): bool =
   interval.years == 0 and interval.months == 0 and