diff --git a/libsrc/apple2/settime.s b/libsrc/apple2/settime.s
index a71e113c5..2235d1a3f 100644
--- a/libsrc/apple2/settime.s
+++ b/libsrc/apple2/settime.s
@@ -29,6 +29,7 @@ _clock_settime:
         tya                     ; Restore A
 
         ; Get tm
+        .assert timespec::tv_sec = 0, error
         jsr     _localtime
         sta     ptr1
         stx     ptr1+1
diff --git a/libsrc/c128/settime.s b/libsrc/c128/settime.s
index afe9c0693..0d95a3958 100644
--- a/libsrc/c128/settime.s
+++ b/libsrc/c128/settime.s
@@ -20,6 +20,7 @@
 
         jsr     pushax
 
+        .assert timespec::tv_sec = 0, error
         jsr     _localtime
         sta     ptr1
         stx     ptr1+1
diff --git a/libsrc/c64/settime.s b/libsrc/c64/settime.s
index b60cb7172..c72d535bc 100644
--- a/libsrc/c64/settime.s
+++ b/libsrc/c64/settime.s
@@ -20,6 +20,7 @@
 
         jsr     pushax
 
+        .assert timespec::tv_sec = 0, error
         jsr     _localtime
         sta     ptr1
         stx     ptr1+1
diff --git a/libsrc/cbm510/settime.s b/libsrc/cbm510/settime.s
index 6e425eaff..0c377eb63 100644
--- a/libsrc/cbm510/settime.s
+++ b/libsrc/cbm510/settime.s
@@ -23,6 +23,7 @@
         jsr     sys_bank
         jsr     pushax
 
+        .assert timespec::tv_sec = 0, error
         jsr     _localtime
         sta     ptr1
         stx     ptr1+1
diff --git a/libsrc/cbm610/settime.s b/libsrc/cbm610/settime.s
index e540c07d0..a42f451cf 100644
--- a/libsrc/cbm610/settime.s
+++ b/libsrc/cbm610/settime.s
@@ -23,6 +23,7 @@
         jsr     sys_bank
         jsr     pushax
 
+        .assert timespec::tv_sec = 0, error
         jsr     _localtime
         sta     ptr1
         stx     ptr1+1