diff --git a/psycopg/pqpath.c b/psycopg/pqpath.c
index 4fb4771e..e87befae 100644
--- a/psycopg/pqpath.c
+++ b/psycopg/pqpath.c
@@ -1616,15 +1616,14 @@ _pq_copy_both_v3(cursorObject *curs)
             ping_time = last_comm;
             ping_time.tv_sec += curs->keepalive_interval;
 
-            if (timercmp(&ping_time, &curr_time, >)) {
-                timersub(&ping_time, &curr_time, &time_diff);
-
+            timersub(&ping_time, &curr_time, &time_diff);
+            if (time_diff.tv_sec > 0) {
                 Py_BEGIN_ALLOW_THREADS;
                 sel = select(PQsocket(conn) + 1, &fds, NULL, NULL, &time_diff);
                 Py_END_ALLOW_THREADS;
             }
             else {
-                sel = 0; /* pretend select() timed out */
+                sel = 0; /* we're past target time, pretend select() timed out */
             }
 
             if (sel < 0) {
diff --git a/psycopg/win32_support.c b/psycopg/win32_support.c
index 8a760b9f..d508b220 100644
--- a/psycopg/win32_support.c
+++ b/psycopg/win32_support.c
@@ -29,6 +29,8 @@
 #include "psycopg/win32_support.h"
 
 #ifdef _WIN32
+
+#ifndef __MINGW32__
 /* millisecond-precision port of gettimeofday for Win32, taken from
    src/port/gettimeofday.c in PostgreSQL core */
 
@@ -58,4 +60,17 @@ gettimeofday(struct timeval * tp, struct timezone * tzp)
 
     return 0;
 }
-#endif /* _WIN32 */
+#endif /* !defined(__MINGW32__) */
+
+/* timersub is missing on mingw */
+void
+timersub(struct timeval *a, struct timeval *b, struct timeval *c)
+{
+    c->tv_sec  = a->tv_sec  - b->tv_sec;
+    c->tv_usec = a->tv_usec - b->tv_usec;
+    if (tv_usec < 0) {
+        c->tv_usec += 1000000;
+        c->tv_sec  -= 1;
+    }
+}
+#endif /* defined(_WIN32) */
diff --git a/psycopg/win32_support.h b/psycopg/win32_support.h
index c6577317..be963df5 100644
--- a/psycopg/win32_support.h
+++ b/psycopg/win32_support.h
@@ -30,7 +30,11 @@
 #include <sys/time.h>
 
 #ifdef _WIN32
+#ifndef __MINGW32__
 HIDDEN int gettimeofday(struct timeval * tp, struct timezone * tzp);
 #endif
 
+HIDDEN void timersub(struct timeval *a, struct timeval *b, struct timeval *c);
+#endif
+
 #endif /* !defined(PSYCOPG_WIN32_SUPPORT_H) */