diff --git a/generic/tclClockFmt.c b/generic/tclClockFmt.c
index 9dd8ecb..188b9bf 100644
--- a/generic/tclClockFmt.c
+++ b/generic/tclClockFmt.c
@@ -56,7 +56,7 @@ static void ClockFrmScnFinalize(ClientData clientData);
  *----------------------------------------------------------------------
  */
 
-/* int overflows may happens here (expected case) */
+/* int & Tcl_WideInt overflows may happens here (expected case) */
 #if defined(__GNUC__) || defined(__GNUG__)
 # pragma GCC optimize("no-trapv")
 #endif
@@ -69,22 +69,33 @@ _str2int(
     const char *e,
     int sign)
 {
-    register int val = 0, prev = 0;
+    register int val = 0;
+    const char *eNO = e;
+    /* overflow impossible for 10 digits ("9..9"), so no needs to check before */
+    if (e-p > 10) {
+	eNO = p+10;
+    }
     if (sign >= 0) {
-	while (p < e) {
+	while (p < eNO) {			/* never overflows */
+	    val = val * 10 + (*p++ - '0');
+	}
+	while (p < e) {				/* check for overflow */
+	    int prev = val;
 	    val = val * 10 + (*p++ - '0');
-	    if (val < prev) {
+	    if (val / 10 < prev) {
 		return TCL_ERROR;
 	    }
-	    prev = val;
 	}
     } else {
-	while (p < e) {
+	while (p < eNO) {			/* never overflows */
+	    val = val * 10 - (*p++ - '0');
+	}
+	while (p < e) {				/* check for overflow */
+	    int prev = val;
 	    val = val * 10 - (*p++ - '0');
-	    if (val > prev) {
+	    if (val / 10 > prev) {
 		return TCL_ERROR;
 	    }
-	    prev = val;
 	}
     }
     *out = val;
@@ -99,22 +110,33 @@ _str2wideInt(
     const char	*e,
     int sign)
 {
-    register Tcl_WideInt val = 0, prev = 0;
+    register Tcl_WideInt val = 0;
+    const char *eNO = e;
+    /* overflow impossible for 18 digits ("9..9"), so no needs to check before */
+    if (e-p > 18) {
+	eNO = p+18;
+    }
     if (sign >= 0) {
-	while (p < e) {
+	while (p < eNO) {			/* never overflows */
+	    val = val * 10 + (*p++ - '0');
+	}
+	while (p < e) {				/* check for overflow */
+	    Tcl_WideInt prev = val;
 	    val = val * 10 + (*p++ - '0');
-	    if (val < prev) {
+	    if (val / 10 < prev) {
 		return TCL_ERROR;
 	    }
-	    prev = val;
 	}
     } else {
-	while (p < e) {
+	while (p < eNO) {			/* never overflows */
+	    val = val * 10 - (*p++ - '0');
+	}
+	while (p < e) {				/* check for overflow */
+	    Tcl_WideInt prev = val;
 	    val = val * 10 - (*p++ - '0');
-	    if (val > prev) {
+	    if (val / 10 > prev) {
 		return TCL_ERROR;
 	    }
-	    prev = val;
 	}
     }
     *out = val;
diff --git a/tests/clock.test b/tests/clock.test
index 513ae94..9f65ac7 100644
--- a/tests/clock.test
+++ b/tests/clock.test
@@ -18685,13 +18685,36 @@ test clock-6.8 {input of seconds} {
 } 9223372036854775807
 
 test clock-6.9 {input of seconds - overflow} {
-    list [catch {clock scan -9223372036854775809 -format %s -gmt true} result] $result $::errorCode
+    list [catch {clock scan -9223372036854775809 -format %s -gmt true} result opt] $result [if {[dict exists $opt -errorcode]} {dict get $opt -errorcode}]
 } {1 {requested date too large to represent} {CLOCK dateTooLarge}}
-
 test clock-6.10 {input of seconds - overflow} {
-    list [catch {clock scan 9223372036854775808 -format %s -gmt true} result] $result $::errorCode
+    list [catch {clock scan 9223372036854775808 -format %s -gmt true} result opt] $result [if {[dict exists $opt -errorcode]} {dict get $opt -errorcode}]
 } {1 {requested date too large to represent} {CLOCK dateTooLarge}}
 
+foreach sign {{} -} {
+  test clock-6.10a {input of seconds - overflow, bug [1f40aa83c5]}  {
+    list [catch {clock scan ${sign}27670116110564327423 -format %s -gmt true} result opt] $result [if {[dict exists $opt -errorcode]} {dict get $opt -errorcode}]
+  } {1 {requested date too large to represent} {CLOCK dateTooLarge}}
+  test clock-6.10b {input of seconds - overflow, bug [1f40aa83c5]} {
+    list [catch {clock scan ${sign}27670116110564327424 -format %s -gmt true} result opt] $result [if {[dict exists $opt -errorcode]} {dict get $opt -errorcode}]
+  } {1 {requested date too large to represent} {CLOCK dateTooLarge}}
+  test clock-6.10c {input of seconds - no overflow, bug [1f40aa83c5]} {
+    list [catch {clock scan ${sign}[string repeat 9 18] -format %s -gmt true} result opt] $result [if {[dict exists $opt -errorcode]} {dict get $opt -errorcode}]
+  } [list 0 ${sign}[string repeat 9 18] {}]
+  test clock-6.10d {input of seconds - overflow, bug [1f40aa83c5]} {
+    list [catch {clock scan ${sign}[string repeat 9 19] -format %s -gmt true} result opt] $result [if {[dict exists $opt -errorcode]} {dict get $opt -errorcode}]
+  } {1 {requested date too large to represent} {CLOCK dateTooLarge}}
+  # both fololowing freescan test don't generate overflow error, 
+  # since it is a free scan, thus the token is simply not recognized further in yacc lexer,
+  # therefore we get parse error (can be surely changed latter):
+  test clock-6.10e {input of seconds - overflow (but since freescan parse error, but not boom), bug [1f40aa83c5]} -body {
+    list [catch {clock scan ${sign}27670116110564327423 -gmt true} result opt] $result [if {[dict exists $opt -errorcode]} {dict get $opt -errorcode}]
+  } -match glob -result {1 {unable to convert date-time string "*": syntax error *} {TCL VALUE DATE PARSE}}
+  test clock-6.10f {input of seconds - overflow (but since freescan parse error, but not boom), bug [1f40aa83c5]} -body {
+    list [catch {clock scan ${sign}27670116110564327424 -gmt true} result opt] $result [if {[dict exists $opt -errorcode]} {dict get $opt -errorcode}]
+  } -match glob -result {1 {unable to convert date-time string "*": syntax error *} {TCL VALUE DATE PARSE}}
+}; unset sign
+
 test clock-6.11 {input of seconds - two values} {
     clock scan {1 2} -format {%s %s} -gmt true
 } 2