errata: add dates

errata.html

@@ -19,14 +19,14 @@ pre {
   <h1>Errata</h1>
   <p>
     <b>p.9, ¶2</b>: for "can compared", read "can be compared".
-    (Thanks to Antonio Macías Ojeda.)
+    (Thanks to Antonio Macías Ojeda, 2015-10-22.)
   </p>
 
   <p>
     <b>p.13</b>: As printed, the <code>gopl.io/ch1/lissajous</code> program
     is deterministic, not random.  We've added the statement below to
     the downloadable program so that it prints a pseudo-random image
-    each time it is run. (Thanks to Randall McPherson.)
+    each time it is run. (Thanks to Randall McPherson, 2015-10-19.)
   </p>
   <pre>rand.Seed(time.Now().UTC().UnixNano())</pre>
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