Legalize Reversed Date

CJam, 24 bytes

qS/Wf%[_1=:=31^sCs].e<S*

I/O format is dd mm yyyy.

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Same byte count, I/O format mm dd yyyy:

qS/Wf%_0=:=1231^s2/.e<S*

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Explanation

qS/     e# Read input, split around spaces.
Wf%     e# Reverse each component.
[       e# Set marker for new list.
  _1=   e#   Duplicate reversed strings, extract reversed month.
  :=    e#   Check for equality of the characters. This gives 1 for
        e#   November (11) and 0 for everything else.
  31^   e#   XOR the result with 31, giving 30 for November and 31
        e#   for everything else.
  s     e#   Convert the result to a string, "30"/"31".
  Cs    e#   Push 12, convert to string, "12".
]       e# Wrap "30"/"31" and "12" in a list.
.e<     e# Element-wise minimum. This clamps the day and month to their
        e# respective maxima.
S*      e# Join the result with spaces.

The other version works similarly, except that we start from the integer 1230 or 1231 before converting it to ["12" "30"] or ["12" "31"].

Pyth, 55 53 46 43 41 bytes

APJ_Mczd=HhS,12sH=GhS,sGC@."❤❤ó»î"H%"%02d %02d %s"[GHeJ
APJ_Mczd=hS,12sH=hS,sGC@."❤❤ó»î"H%"%02d %02d %s"[GHeJ
APJ_Mcz\-%"%02d %02d %s"[hS,sGx31q11sHhS,12sHeJ
APJ_Mczdjd[>2+\0hS,sGx31q11sH>2+\0hS,12sHeJ
APJ_Mczdjd.[L\02[`hS,sGx31q11sH`hS,12sHeJ

where ❤❤ were two unprintables, respectively U+001C and U+001F.

Test suite.

Convex, 23 bytes

Byte count assumes CP-1252 encoding.

qS/Wf%_1=:=31^sCs¶.e<S*

I/O format is dd mm yyyy.

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This is a direct port of my CJam answer. Convex is heavily based on CJam, and therefore the only difference is the use of Convex's ¶ operator which wraps the top two stack elements in a list, saving a byte over [...].