diff --git a/app/business/__init__.py b/app/business/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/app/business/time_calc.py b/app/business/time_calc.py
new file mode 100644
index 0000000..63c4260
--- /dev/null
+++ b/app/business/time_calc.py
@@ -0,0 +1,25 @@
+def minutes_to_str(minutes: int) -> str:
+    sign = "-" if minutes < 0 else ""
+    minutes = abs(minutes)
+    return f"{sign}{minutes // 60}h{minutes % 60:02d}"
+
+
+_REFERENCE_MINUTES = {
+    "WORK": 465,
+    "TT": 465,
+    "FORMATION": 465,
+    "GARDE": 600,
+    "ASTREINTE": 0,
+    "RTT": 0,
+    "CONGE": 0,
+    "MALADE": 0,
+    "FERIE": 0,
+}
+
+
+def work_minutes_reference(day_type: str) -> int:
+    return _REFERENCE_MINUTES.get(day_type, 465)
+
+
+def week_balance_minutes(actual_minutes: int, reference_minutes: int) -> int:
+    return actual_minutes - reference_minutes
diff --git a/tests/test_time_calc.py b/tests/test_time_calc.py
new file mode 100644
index 0000000..87f24c6
--- /dev/null
+++ b/tests/test_time_calc.py
@@ -0,0 +1,42 @@
+from app.business.time_calc import (
+    minutes_to_str,
+    work_minutes_reference,
+    week_balance_minutes,
+)
+
+
+def test_minutes_to_str_basic():
+    assert minutes_to_str(465) == "7h45"
+
+
+def test_minutes_to_str_zero():
+    assert minutes_to_str(0) == "0h00"
+
+
+def test_minutes_to_str_negative():
+    assert minutes_to_str(-75) == "-1h15"
+
+
+def test_work_minutes_reference_normal_day():
+    assert work_minutes_reference("WORK") == 465
+    assert work_minutes_reference("TT") == 465
+    assert work_minutes_reference("FORMATION") == 465
+
+
+def test_work_minutes_reference_garde():
+    assert work_minutes_reference("GARDE") == 600
+
+
+def test_work_minutes_reference_absence():
+    assert work_minutes_reference("RTT") == 0
+    assert work_minutes_reference("CONGE") == 0
+    assert work_minutes_reference("MALADE") == 0
+    assert work_minutes_reference("FERIE") == 0
+
+
+def test_week_balance_positive():
+    assert week_balance_minutes(2400, 2325) == 75
+
+
+def test_week_balance_negative():
+    assert week_balance_minutes(2200, 2325) == -125