diff --git a/pythonetc/README.md b/pythonetc/README.md
index b9bc52f..c13a2a9 100644
--- a/pythonetc/README.md
+++ b/pythonetc/README.md
@@ -67,6 +67,7 @@ More:
 1. ./qualname.md                (24 November 2020, 18:00)
 1. ./digits.md                  (26 November 2020, 18:00)
 1. ./emoji.md                   (1 December 2020, 18:00)
+1. ./json-default.md
 1. ./ipython.md
 1. ./array.md
 
@@ -78,7 +79,6 @@ Out of order:
 ## TODO
 
 Json indent
-JSON custom loader
 Csv instead of xls
 Turtle
 Bytearray
@@ -90,5 +90,4 @@ String.Template
 String module consts
 Urllib
 Always compile regexp
-Re.Verbose
 Tau vs pi
diff --git a/pythonetc/json-default.md b/pythonetc/json-default.md
new file mode 100644
index 0000000..23fd9d9
--- /dev/null
+++ b/pythonetc/json-default.md
@@ -0,0 +1,44 @@
+
+`json.dumps` can serialize every built-in type which has a corresponding JSON type (`int` as `number`, `None` as `null`, `list` as `array` etc) but fails for every other type. Probably, the most often case when you will face it is when trying to serialize a datetime object:
+
+```python
+import json
+from datetime import datetime
+
+json.dumps([123, 'hello'])
+# '[123, "hello"]'
+
+json.dumps(datetime.now())
+# TypeError: Object of type 'datetime' is not JSON serializable
+```
+
+The fastest way to fix it is to provide a custom default serializer:
+
+```python
+json.dumps(datetime.now(), default=str)
+# '"2020-12-03 18:00:10.592496"'
+```
+
+However, that means that every unknown object will be serialized into a string which can lead to unexpected result:
+
+```python
+class C: pass
+json.dumps(C(), default=str)
+'"<__main__.C object at 0x7f330ec801d0>"'
+```
+
+So, if you want to serialize only `datetime` and nothing else, it's better to define a custom encoder:
+
+```python
+class DateTimeEncoder(json.JSONEncoder):
+  def default(self, obj) -> str:
+    if isinstance(obj, datetime):
+      return obj.isoformat()
+    return super().default(obj)
+
+json.dumps(datetime.now(), cls=DateTimeEncoder)
+'"2020-12-03T18:01:19.609648"'
+
+json.dumps(C(), cls=DateTimeEncoder)
+# TypeError: Object of type 'C' is not JSON serializable
+```