diff --git a/latency_calculator/tests/test_core.py b/latency_calculator/tests/test_core.py
new file mode 100644
index 0000000..88c615c
--- /dev/null
+++ b/latency_calculator/tests/test_core.py
@@ -0,0 +1,85 @@
+import pytest
+from datetime import datetime, timedelta
+from latency_calculator import core
+
+
+def create_timestamp_entry(delta_publish_to_gate: float, delta_gate_to_visible: float):
+    base_time = datetime(2024, 1, 1, 12, 0, 0)
+    t_publish = base_time
+    t_gate_read = base_time + timedelta(seconds=delta_publish_to_gate)
+    t_index_visible = t_gate_read + timedelta(seconds=delta_gate_to_visible)
+    return {
+        't_publish': t_publish.isoformat(),
+        't_gate_read': t_gate_read.isoformat(),
+        't_index_visible': t_index_visible.isoformat(),
+    }
+
+
+def test_calculate_latency_typical_case():
+    entries = [
+        create_timestamp_entry(2, 3),
+        create_timestamp_entry(1, 1),
+        create_timestamp_entry(4, 2),
+        create_timestamp_entry(3, 4),
+        create_timestamp_entry(0.5, 0.5),
+    ]
+
+    result = core.calculate_latency(entries)
+
+    assert isinstance(result, dict)
+    assert set(result.keys()) == {'p50', 'p95', 'max'}
+
+    # Latency values should be positive numbers
+    assert all(result[key] >= 0 for key in result)
+
+
+def test_calculate_latency_edge_case_single_entry():
+    entry = [create_timestamp_entry(1, 1)]
+    result = core.calculate_latency(entry)
+    expected = entry[0]
+    assert 'p50' in result and 'p95' in result and 'max' in result
+    assert result['p50'] == pytest.approx(result['max'], rel=1e-9)
+
+
+def test_calculate_latency_invalid_input():
+    # Missing required keys
+    bad_entry = [{ 't_publish': '2024-01-01T00:00:00' }]
+    with pytest.raises((KeyError, ValueError, TypeError)):
+        core.calculate_latency(bad_entry)
+
+
+def test_calculate_latency_mixed_timestamp_formats():
+    base_time = datetime(2024, 1, 1, 12, 0, 0)
+    entry_1 = {
+        't_publish': base_time.timestamp(),
+        't_gate_read': (base_time + timedelta(seconds=2)).timestamp(),
+        't_index_visible': (base_time + timedelta(seconds=5)).timestamp(),
+    }
+    entry_2 = {
+        't_publish': (base_time + timedelta(seconds=10)).isoformat(),
+        't_gate_read': (base_time + timedelta(seconds=12)).isoformat(),
+        't_index_visible': (base_time + timedelta(seconds=16)).isoformat(),
+    }
+
+    result = core.calculate_latency([entry_1, entry_2])
+
+    assert isinstance(result, dict)
+    assert result['p50'] > 0
+    assert result['max'] >= result['p95'] >= result['p50']
+
+
+def test_calculate_latency_outlier_influence():
+    normal_entries = [create_timestamp_entry(1, 1) for _ in range(9)]
+    outlier = create_timestamp_entry(10, 20)
+    entries = normal_entries + [outlier]
+    result = core.calculate_latency(entries)
+
+    assert result['max'] == pytest.approx(30.0, rel=1e-9)
+    # median should remain near normal values
+    assert result['p50'] < 5.0
+
+
+def test_calculate_latency_assertions_strict_types():
+    # Passing list instead of dict should fail
+    with pytest.raises((TypeError, ValueError)):
+        core.calculate_latency([['wrong_type']])
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