Improve types for some date operations; fix typo.

original commit: e67415171a4655f1fa1f399abebb8e7d47abcf31

collects/typed-scheme/base-env/base-env.rkt

@@ -685,7 +685,8 @@
 
 
 
-[seconds->date (-Integer . -> . (make-Name #'date))]
+[seconds->date (cl->* (-Integer . -> . (make-Name #'date))
+                      (-Integer Univ . -> . (make-Name #'date)))]
 [current-seconds (-> -Integer)]
 
 ;Section 14.2

collects/typed-scheme/base-env/base-structs.rkt

@@ -50,16 +50,16 @@
      [span : (*Un -Integer (-val #f))]))
 
   (define-hierarchy date (#:kernel-maker k:date)
-    ([second : -Number]
-     [minute : -Number]
-     [hour : -Number]
-     [day : -Number]
-     [month : -Number]
-     [year : -Number]
-     [weekday : -Number]
-     [year-day : -Number]
+    ([second : -Nat]
+     [minute : -Nat]
+     [hour : -Nat]
+     [day : -Nat]
+     [month : -Nat]
+     [year : -Nat]
+     [week-day : -Nat]
+     [year-day : -Nat]
      [dst? : -Boolean]
-     [time-zone-offset : -Number]))
+     [time-zone-offset : -Integer]))
 
   (define-hierarchy arity-at-least (#:kernel-maker k:arity-at-least)
     ([value : -Nat]))