old_contents: from django.contrib.auth.models import User, Group
  from rest_framework import serializers
  
  class UnixEpochDateField(serializers.DateTimeField):
      def to_native(self, value):
          """ Return epoch time for a datetime object or ``None``"""
          import time
          try:
              return int(time.mktime(value.timetuple()))
          except (AttributeError, TypeError):
              return None
  
      def from_native(self, value):
          import datetime
          return datetime.datetime.fromtimestamp(int(value))
          
  
  class UserSerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = User
          fields = ('url', 'username', 'email', 'groups')
  
  class GroupSerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = Group
          fields = ('url', 'name')
  
  from msg.models import Msg
  
  class MsgSerializer( serializers.ModelSerializer ):
      
      class Meta:
          model = Msg
          epoch = UnixEpochDateField(source='timestamp')
          fields = ('frame_id','timestamp','source','channel','signature','body')
          read_only_fields = ('epoch',)
  
new_contents: from django.contrib.auth.models import User, Group
  from rest_framework import serializers
  
  class UnixEpochDateField(serializers.DateTimeField):
      def to_native(self, value):
          """ Return epoch time for a datetime object or ``None``"""
          import time
          try:
              return int(time.mktime(value.timetuple()))
          except (AttributeError, TypeError):
              return None
  
      def from_native(self, value):
          import datetime
          return datetime.datetime.fromtimestamp(int(value))
          
  
  class UserSerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = User
          fields = ('url', 'username', 'email', 'groups')
  
  class GroupSerializer(serializers.HyperlinkedModelSerializer):
      class Meta:
          model = Group
          fields = ('url', 'name')
  
  from msg.models import Msg
  
  class MsgSerializer( serializers.ModelSerializer ):
      epoch = UnixEpochDateField(source='timestamp')
          
      class Meta:
          model = Msg
          fields = ('frame_id','timestamp','source','channel','signature','body','epoch')
   
  
subject: Update serializers.py