From 3bddaae76d217ffaba7130d0a68f4f4e0762b12e Mon Sep 17 00:00:00 2001
From: Nick Walker <nick@nickwalker.us>
Date: Fri, 26 Jun 2026 13:30:33 -0600
Subject: [PATCH] Fix to_systemtime sub-second precision for non-standard NTSC
 rates

System time is defined in the integer frame grid, so the sub-second
calculation should always divide by _int_framerate. Using
float(framerate) was imprecise for non-standard NTSC rates stored as
rounded strings (e.g. "47.95" for 48000/1001), producing wrong
millisecond values mid-second.

Co-authored-by: cubicibo <55701024+cubicibo@users.noreply.github.com>
---
 src/timecode/timecode.py |  8 ++++----
 tests/test_timecode.py   | 15 +++++++++++++++
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/src/timecode/timecode.py b/src/timecode/timecode.py
index 6a0e2b3..9722e0a 100644
--- a/src/timecode/timecode.py
+++ b/src/timecode/timecode.py
@@ -422,10 +422,10 @@ def to_systemtime(self, as_float: bool = False) -> str | float:  # type:ignore
             return self.float - (1e-3) if as_float else str(self)
 
         hh, mm, ss, ff = self.frames_to_tc(self.frames + 1, skip_rollover=True)
-        framerate = (
-            float(self.framerate) if self._ntsc_framerate else self._int_framerate
-        )
-        ms = ff / framerate
+        # System time is in the integer frame grid, so always divide by _int_framerate.
+        # Using float(self.framerate) for NTSC rates gives wrong sub-second values
+        # because the stored string (e.g. "47.95") loses precision vs the true rate.
+        ms = ff / self._int_framerate
         if as_float:
             return hh * 3600 + mm * 60 + ss + ms
         return f"{hh:02d}:{mm:02d}:{ss:02d}.{round(ms * 1000):03d}"
diff --git a/tests/test_timecode.py b/tests/test_timecode.py
index 99efb52..73a9a6c 100644
--- a/tests/test_timecode.py
+++ b/tests/test_timecode.py
@@ -1764,3 +1764,18 @@ def test_generalized_ntsc_rational_formats(rational_str, int_framerate, is_drop)
     assert tc._ntsc_framerate is True
     assert tc._int_framerate == int_framerate
     assert tc.drop_frame is is_drop
+
+
+def test_systemtime_subframe_ntsc():
+    """to_systemtime uses integer framerate for sub-second calculation.
+
+    Non-standard NTSC rates (e.g. 48000/1001) are stored as a rounded float
+    string ("47.95"), so dividing by float(framerate) gives wrong milliseconds
+    for frames mid-second. The integer framerate (48) must be used instead.
+    """
+    from fractions import Fraction
+    # Frame 24 of 48000/1001 fps is exactly half a second in system time (24/48)
+    tc = Timecode(Fraction(48000, 1001), frames=24)
+    assert tc._ntsc_framerate is True
+    assert tc._int_framerate == 48
+    assert tc.to_systemtime() == "00:00:00.500"