Use more robust logic in UpdateSun.c

util/UpdateSun.c
- Update the computation to avoid integer division, which can lead
  to non-deterministic results (AI generated fix)
- Ensure that Tlocal is in the range 0..24

CHANGELOG.md
- Updated accordingly

Signed-off-by: Bob Yantosca <[email protected]>

Author: yantosca

CHANGELOG.md

@@ -27,6 +27,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed a bug that prevented `.ci-pipelines/ci-cleanup-script.sh` from removing KPP-generated files for MCM mechanisms
 - Fixed typo in error message in `int/rosenbrock_autoreduce.f90`
 - Fixed several issues in `int/runge_kutta.c` that were causing numerical instability or non-deterministic results
+- Fixed computation of sun angle in `util/UpdateSun.c` to ensure deterministic behavior
 
 ### Removed
 - Removed C-I tests on Microsoft Azure Dev Pipelines

util/UpdateSun.c

@@ -2,12 +2,30 @@ void Update_SUN()
 {
 KPP_REAL SunRise, SunSet;
 KPP_REAL Thour, Tlocal, Ttmp; 
+KPP_REAL days;
 const KPP_REAL PI = 3.14159265358979;  
 
   SunRise = 4.5;
   SunSet  = 19.5;
   Thour = TIME/3600.0;
-  Tlocal = Thour - ((int)Thour/24)*24;
+  // Calculate integer number of days, handling both positive and negative correctly
+  // Use explicit floor operation: for positive values, truncation works;
+  // for negative, we need floor behavior
+  // Fix generated by AI and implemented by @yantosca, 31 Oct 2025
+  if (Thour >= 0.0) {
+    days = (KPP_REAL)((long)(Thour / 24.0));
+  } else {
+    /* For negative, subtract 1 to get floor behavior */
+    days = (KPP_REAL)((long)(Thour / 24.0));
+    if ((Thour - days*24.0) < 0.0) {
+      days -= 1.0;
+    }
+  }
+  Tlocal = Thour - days*24.0;
+  // Ensure Tlocal is in [0, 24) range for determinism
+  // Fix generated by AI and implemented by @yantosca, 31 Oct 2025
+  if (Tlocal >= 24.0) Tlocal -= 24.0;
+  if (Tlocal < 0.0) Tlocal += 24.0;
 
   if ( (Tlocal >= SunRise) && (Tlocal <= SunSet) ) {
     Ttmp = (2.0*Tlocal-SunRise-SunSet)/(SunSet-SunRise);