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web/core/modules/serialization/config/install/serialization.settings.yml

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+# Before Drupal 8.3, typed data primitive values were normalized as strings, as
+# this was usually returned from database storage. A primitive data normalizer
+# has been introduced to get the casted value instead.
+bc_primitives_as_strings: false
+# Before Drupal 8.3, timestamps were always returned as Unix timestamps, which
+# are not a universal format for interchange. Now, RFC3339 timestamps are
+# returned. New Drupal installations opt out from this by default (hence this
+# is set to false), existing installations opt in to it.
+# @see serialization_update_8301()
+# @see https://www.drupal.org/node/2768651
+bc_timestamp_normalizer_unix: false

web/core/modules/serialization/config/schema/serialization.schema.yml

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+serialization.settings:
+  type: config_object
+  label: 'Serialization settings'
+  mapping:
+    bc_primitives_as_strings:
+      type: boolean
+      label: 'Whether to retain pre Drupal 8.3 behavior of serializing all primitive items as strings.'
+    bc_timestamp_normalizer_unix:
+      type: boolean
+      label: 'Whether the pre Drupal 8.4 behavior of returning Unix timestamps instead of RFC3339 timestamps for TimestampItem fields is enabled or not.'