diff --git a/debian/patches/01-fix-pre-ansi-function-declarations.patch b/debian/patches/01-fix-pre-ansi-function-declarations.patch
new file mode 100644
index 0000000..f96891a
--- /dev/null
+++ b/debian/patches/01-fix-pre-ansi-function-declarations.patch
@@ -0,0 +1,55 @@
+Author: Jeremy Sowden <jeremy@azazel.net>
+Last-Update: 2025-02-27
+Bug-Debian: https://bugs.debian.org/1098100
+Forwarded: yes
+Applied-Upstream: commit:97d7faf9150414ddfc3a53cd9905e628dd60685f
+Description: remove pre-ANSI function declarations
+ Pre-ANSI function declarators which do not declare their parameters have been
+ removed in C23 and the syntax reused for declaring functions with no parameters
+ (like C++).  GCC 15 defaults to C23, so these declarators will cause an FTBFS.
+ .
+ Update the function definitions to have ANSI prototypes where these are
+ missing, make them static and replace the obsolete declarations with modern
+ equivalents.
+
+--- a/Src/wmCalClock.c
++++ b/Src/wmCalClock.c
+@@ -119,6 +119,8 @@
+ 
+ 
+ 
++static double jd(int ny, int nm, int nd, double UT);
++static double hour24(double hour);
+ 
+ void ParseCMDLine(int argc, char *argv[]);
+ void ButtonPressEvent(XButtonEvent *);
+@@ -310,7 +312,7 @@
+     int 		Year, Month, DayOfWeek, DayOfMonth, OldDayOfMonth;
+     int			Hours, Mins, Secs, OldSecs, digit, xoff, D[10], xsize;
+     time_t		CurrentLocalTime;
+-    double		UT, TU, TU2, TU3, T0, gmst, jd(), hour24();
++    double		UT, TU, TU2, TU3, T0, gmst;
+ 
+ 
+     /*
+@@ -939,9 +941,7 @@
+  *  Compute the Julian Day number for the given date.
+  *  Julian Date is the number of days since noon of Jan 1 4713 B.C.
+  */
+-double jd(ny, nm, nd, UT)
+-int ny, nm, nd;
+-double UT;
++static double jd(int ny, int nm, int nd, double UT)
+ {
+         double A, B, C, D, JD, day;
+ 
+@@ -976,8 +976,7 @@
+ 
+ }
+ 
+-double hour24(hour)
+-double hour;
++static double hour24(double hour)
+ {
+         int n;
+ 
diff --git a/debian/patches/series b/debian/patches/series
new file mode 100644
index 0000000..50e3424
--- /dev/null
+++ b/debian/patches/series
@@ -0,0 +1 @@
+01-fix-pre-ansi-function-declarations.patch