Handle omitted Act/365L reference end date

financepy/utils/day_count.py

@@ -255,9 +255,9 @@ def year_frac(
             frequency = annual_frequency(freq_type)
 
             if dt3 is None:
-                y3 = y2
-            else:
-                y3 = dt3.y
+                dt3 = dt2
+
+            y3 = dt3.y
 
             num = dt2 - dt1
             den = 365

unit_tests/test_FinDayCount.py

@@ -120,6 +120,52 @@ def test_year_frace_act_365_l():
 ########################################################################################
 
 
+def test_act_365_l_without_reference_end_date():
+
+    day_count = DayCount(DayCountTypes.ACT_365L)
+    period_start = Date(1, 12, 2023)
+    period_end = Date(1, 3, 2024)
+
+    implicit_period_end = day_count.year_frac(
+        period_start,
+        period_end,
+        freq_type=FrequencyTypes.ANNUAL,
+    )
+    explicit_period_end = day_count.year_frac(
+        period_start,
+        period_end,
+        period_end,
+        FrequencyTypes.ANNUAL,
+    )
+
+    assert implicit_period_end == explicit_period_end
+    assert implicit_period_end == (91.0 / 366.0, 91.0, 366)
+
+    # A separately supplied coupon-period end must still control the
+    # denominator for an accrued fraction.
+    accrued_start = Date(1, 3, 2023)
+    accrued_end = Date(1, 12, 2023)
+    next_coupon = Date(1, 3, 2024)
+
+    full_period_mode = day_count.year_frac(
+        accrued_start,
+        accrued_end,
+        freq_type=FrequencyTypes.ANNUAL,
+    )
+    accrued_mode = day_count.year_frac(
+        accrued_start,
+        accrued_end,
+        next_coupon,
+        FrequencyTypes.ANNUAL,
+    )
+
+    assert full_period_mode == (275.0 / 365.0, 275.0, 365)
+    assert accrued_mode == (275.0 / 366.0, 275.0, 366)
+
+
+########################################################################################
+
+
 def test_year_frace_simple():
 
     dc_type = DayCountTypes.JULIAN