allow negative timestamps on content import

closes #4551

Change-Id: I39772343d2a33cd6a0c6e08457d32ce871c9932c
Reviewed-on: https://gerrit.instructure.com/3598
Reviewed-by: Brian Palmer <brianp@instructure.com>
Reviewed-by: Zach Wily <zach@instructure.com>
Tested-by: Hudson <hudson@instructure.com>

Author: bracken

lib/canvas_migration/migrator_helper.rb

@@ -32,16 +32,16 @@ module Canvas::MigratorHelper
   def self.get_utc_time_from_timestamp(timestamp)
     # timestamp can be either a time string in the format "2011-04-30T00:00:00-06:00",
     # or an integer epoch * 1000
-    if timestamp.to_s.match(/[^0-9.]/)
-      Time.parse(timestamp.to_s)
-    else
+    if timestamp.to_s.match(/^-?[0-9.]+$/)
       timestamp = timestamp.to_i/ 1000 rescue 0
       t = nil
       if timestamp > 0
         t = Time.at(timestamp)
         t = Time.utc(t.year, t.month, t.day, t.hour, t.min, t.sec)
       end
       t
+    else
+      Time.parse(timestamp.to_s)
     end
   end