diff --git a/lib/groupdate/magic.rb b/lib/groupdate/magic.rb
index d5a325f..26ce88e 100644
--- a/lib/groupdate/magic.rb
+++ b/lib/groupdate/magic.rb
@@ -85,6 +85,10 @@ module Groupdate
           raise "Connection adapter not supported: #{adapter_name}"
         end
 
+      if adapter_name == "MySQL" && field == :week
+        query[0] = "CAST(#{query[0]} AS DATETIME)"
+      end
+
       group = relation.group(Groupdate::OrderHack.new(relation.send(:sanitize_sql_array, query), field, time_zone))
       if options[:series] == false
         group
@@ -126,7 +130,7 @@ module Groupdate
           lambda { |k| k.to_i }
         else
           utc = ActiveSupport::TimeZone["UTC"]
-          lambda { |k| (k.is_a?(String) ? utc.parse(k) : k.to_time).in_time_zone(time_zone) }
+          lambda { |k| (k.is_a?(String) ? utc.parse(k) : (k.is_a?(BigDecimal) ? Time.at(k) : k.to_time)).in_time_zone(time_zone) }
         end
 
       count =
diff --git a/test/test_helper.rb b/test/test_helper.rb
index cd82e23..42e4f87 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -864,6 +864,7 @@ module TestGroupdate
 
   def test_zeros_null_value
     create_user nil
+    User.update_all created_at: nil, created_on: nil # for MySQL adapter
     assert_equal 0, call_method(:hour_of_day, :created_at, range: true)[0]
   end
 
--
libgit2 0.21.0