Add a trading day timedelta

test/test_trading_days.py

@@ -0,0 +1,17 @@
+from unittest import TestCase
+from datetime import datetime
+
+from metrik.trading_days import TradingDay
+
+
+class TradingDayTest(TestCase):
+
+    def test_skip_july4(self):
+        start = datetime(2016, 7, 1)  # Friday
+        end = start + TradingDay(1)
+        assert end == datetime(2016, 7, 5)
+
+    def test_skip_july4_backwards(self):
+        end = datetime(2016, 7, 5)
+        start = end - TradingDay(1)
+        assert start == datetime(2016, 7, 1)