diff --git a/exercises/compareTime.sml b/exercises/compareTime.sml
index 4489c57..ec02261 100644
--- a/exercises/compareTime.sml
+++ b/exercises/compareTime.sml
@@ -1,4 +1,8 @@
-fun timeBefore((h1, m1, "AM"), (h2, m2, "AM")) = false;
+fun timeBefore((h1, m1, "AM"), (h2, m2, "AM")) =
+    (h1 < h2) orelse (h1 = h2 andalso m1 < m2)
+  | timeBefore((h1, m1, "PM"), (h2, m2, "PM")) =
+    (h1 < h2) orelse (h1 = h2 andalso m1 < m2)
+  | timeBefore _ = false;
 
 timeBefore((1, 59, "AM"), (2, 40, "PM")) = true;
 timeBefore((1, 59, "PM"), (2, 40, "PM")) = true;
diff --git a/exercises/fib.sml b/exercises/fib.sml
index 705bab2..711f7bb 100644
--- a/exercises/fib.sml
+++ b/exercises/fib.sml
@@ -1,6 +1,8 @@
-fun fib(0) = 0
-| fib(1) = 1
-| fib(n) = fib(n-2) + fib(n-1);
+fun fibr(a, b, 0) = a
+| fibr(a, b, 1) = b
+| fibr(a, b, count) = fibr(b, a+b, count-1);
+
+fun fib(n) = fibr(0, 1, n);
 
 fib(10) = 55;
 fib(8) = 21;
diff --git a/exercises/max.sml b/exercises/max.sml
index bed83eb..3780150 100644
--- a/exercises/max.sml
+++ b/exercises/max.sml
@@ -1,5 +1,6 @@
 fun maxm [m] = m
-| maxm(m::n::ns) = if m > n then maxm(m::ns) else maxm(n::ns);
+| maxm(m::n::ns) = if m > n then maxm(m::ns) else maxm(n::ns)
+| maxm([]) = 0;
 
 maxm([43,25,2,4,2434]);
 maxm([]);
\ No newline at end of file
diff --git a/exercises/prod.sml b/exercises/prod.sml
index 8987833..a49e0b5 100644
--- a/exercises/prod.sml
+++ b/exercises/prod.sml
@@ -1,6 +1,9 @@
 (* Make tail recursive *)
-fun prod [] = 1
-| prod (n::ns) = n * (prod ns);
+
+fun prod l = prodr(l, 1);
 
 prod([2,3,4,5]) = 120;
-prod([]) = 1;
\ No newline at end of file
+prod([]) = 1;
+
+fun prodr([], result) = result
+| prodr ((n::ns), r) = prodr(ns, n+r);
\ No newline at end of file
diff --git a/exercises/validateDayMonth.sml b/exercises/validateDayMonth.sml
index 609af5a..50fb264 100644
--- a/exercises/validateDayMonth.sml
+++ b/exercises/validateDayMonth.sml
@@ -1,2 +1,16 @@
 (* Complete the expression *)
-fun isValidDate(d, m) = true;
\ No newline at end of file
+fun isValidDate(d, m) = 
+    case m of
+        "January" => d >= 1 andalso d <= 31
+        | "February" =>
+        | "March" =>
+        | "April" =>
+        | "May" =>
+        | "June" =>
+        | "July" =>
+        | "August" =>
+        | "September" =>
+        | "October" =>
+        | "November" =>
+        | "December" =>
+        | _ => false
\ No newline at end of file