diff --git a/.github/workflows/python-tests.yml b/.github/workflows/python-tests.yml
new file mode 100644
index 0000000..c7d9102
--- /dev/null
+++ b/.github/workflows/python-tests.yml
@@ -0,0 +1,26 @@
+name: Python package
+
+on: [push]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.8", "3.10"]
+
+    steps:
+      - uses: actions/checkout@v3
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        # This case we are only updating pip, but you could add other dependencies needed.
+        run: |
+          python -m pip install --upgrade pip
+          python3 -m pip install pytest
+      - name: Test with pytest
+        run: |
+          pytest
diff --git a/__pycache__/test_times.cpython-37-pytest-6.1.2.pyc b/__pycache__/test_times.cpython-37-pytest-6.1.2.pyc
new file mode 100644
index 0000000..c7023c4
Binary files /dev/null and b/__pycache__/test_times.cpython-37-pytest-6.1.2.pyc differ
diff --git a/__pycache__/times.cpython-37.pyc b/__pycache__/times.cpython-37.pyc
new file mode 100644
index 0000000..249ca7f
Binary files /dev/null and b/__pycache__/times.cpython-37.pyc differ
diff --git a/test_times.py b/test_times.py
new file mode 100644
index 0000000..25dc96d
--- /dev/null
+++ b/test_times.py
@@ -0,0 +1,44 @@
+# Test Times File
+
+from times import compute_overlap_time
+from times import time_range
+import pytest
+
+def test_subset_overlap():
+    large = time_range("2010-01-12 10:00:00", "2010-01-12 12:00:00", 2)
+    short = time_range("2010-01-12 10:30:00", "2010-01-12 10:45:00", 2)
+    result = compute_overlap_time(large, short)    
+    expected = 15.0*60.0
+    assert result == expected
+    
+def test_partial_overlap():
+    large = time_range("2010-01-12 10:00:00", "2010-01-12 10:40:00", 2)
+    short = time_range("2010-01-12 10:30:00", "2010-01-12 10:45:00", 2)
+
+    result = compute_overlap_time(large, short)    
+    expected = 10.0*60.0
+    assert result == expected
+    
+def test_vanishing_overlap():
+    large = time_range("2010-01-12 10:45:00", "2010-01-12 12:00:00", 2)
+    short = time_range("2010-01-12 10:30:00", "2010-01-12 10:45:00", 2)
+
+    result = compute_overlap_time(large, short)    
+    expected = 0.0
+    assert result == expected
+
+def test_zero_overlap():
+    large = time_range("2010-01-12 12:00:00", "2010-01-12 12:00:00", 2)
+    short = time_range("2010-01-12 10:30:00", "2010-01-12 10:45:00", 2,)
+
+    result = compute_overlap_time(large, short)    
+    expected = 0.0
+    assert result == expected
+    
+def test_input_values():
+    with pytest.raises(ValueError):
+        large = time_range("2010-01-12 10:00:00", "2010-01-12 12:00:00", 2)
+        short = time_range("2010-01-12 10:50:00", "2010-01-12 10:45:00", 2)
+        result = compute_overlap_time(large, short)
+
+# Forgot to mention Answers UCL-COMP0233-22-23/RSE-Classwork#19
diff --git a/times.py b/times.py
index d57f401..f946903 100644
--- a/times.py
+++ b/times.py
@@ -1,7 +1,13 @@
 import datetime
+import time
+from pytest import raises
 
 
-def time_range(start_time, end_time, number_of_intervals=1, gap_between_intervals_s=0):
+def correctly_ordered_inputs(range0):
+    if range0[0][0] > range0[-1][1]:
+        raise ValueError("\n The input contains a time range which goes backwards in time as index is increased. \n Please ensure inputs are time-ordered.")
+
+def time_range(start_time, end_time, number_of_intervals=1, gap_between_intervals_s=0):  
     start_time_s = datetime.datetime.strptime(start_time, "%Y-%m-%d %H:%M:%S")
     end_time_s = datetime.datetime.strptime(end_time, "%Y-%m-%d %H:%M:%S")
     d = (end_time_s - start_time_s).total_seconds() / number_of_intervals + gap_between_intervals_s * (1 / number_of_intervals - 1)
@@ -11,16 +17,27 @@ def time_range(start_time, end_time, number_of_intervals=1, gap_between_interval
     return [(ta.strftime("%Y-%m-%d %H:%M:%S"), tb.strftime("%Y-%m-%d %H:%M:%S")) for ta, tb in sec_range]
 
 
+
 def compute_overlap_time(range1, range2):
-    overlap_time = []
-    for start1, end1 in range1:
-        for start2, end2 in range2:
-            low = max(start1, start2)
-            high = min(end1, end2)
-            overlap_time.append((low, high))
-    return overlap_time
+    
+    # Checking inputs
+    correctly_ordered_inputs(range1)
+    correctly_ordered_inputs(range2)
+    
+    latest_start = max(min([start for start, end in range1]), 
+                       min([start for start, end in range2]))
+    earliest_end = min(max([end for start, end in range1]), 
+                       max([end for start, end in range2]))
+    ls_sec = time.mktime(time.strptime(latest_start, "%Y-%m-%d %H:%M:%S"))
+    ee_sec = time.mktime(time.strptime(earliest_end, "%Y-%m-%d %H:%M:%S"))
+    if ls_sec < ee_sec:
+        return ee_sec - ls_sec
+    else:
+        return 0
+    
 
 if __name__ == "__main__":
-    large = time_range("2010-01-12 10:00:00", "2010-01-12 12:00:00")
-    short = time_range("2010-01-12 10:30:00", "2010-01-12 10:45:00", 2, 60)
-    print(compute_overlap_time(large, short))
\ No newline at end of file
+    large = time_range("2010-01-12 10:35:00", "2010-01-12 10:31:00", 2)
+    short = time_range("2010-01-12 10:30:00", "2010-01-12 10:50:00", 2)
+    print()
+    print(compute_overlap_time(large, short), 'seconds of overlap')
\ No newline at end of file