Don't know if it helps anyone - but I created a function similar to checkdate() called checktime() that can be used like this:

bool checktime(int $hour, int $minute [, int $second [, bool $natime ]]);

$natime defaults to FALSE, but if TRUE, the function assumes North-American style hours are used - so only 1 - 12 is valid (and not 0 - 23).

So:
checktime(23, 12)           // true - 23:12 is ok
checktime(24, 61, 67)       // false - no such time 24:61:67
checktime(18, 43, 30, true) // false - NA Time only allows hour 01 - 12
etc.

Patch against PHP 5.2 HEAD follows.

Shahar.

--- snip ---

Index: ext/date/php_date.c
===================================================================
RCS file: /repository/php-src/ext/date/php_date.c,v
retrieving revision 1.43.2.45.2.51
diff -u -r1.43.2.45.2.51 php_date.c
--- ext/date/php_date.c 12 Jul 2007 18:59:05 -0000      1.43.2.45.2.51
+++ ext/date/php_date.c 10 Aug 2007 17:04:21 -0000
@@ -83,6 +83,14 @@
 ZEND_END_ARG_INFO()
 
 static
+ZEND_BEGIN_ARG_INFO_EX(arginfo_checktime, 0, 0, 2)
+       ZEND_ARG_INFO(0, hour)
+       ZEND_ARG_INFO(0, minute)
+       ZEND_ARG_INFO(0, second)
+       ZEND_ARG_INFO(0, natime)
+ZEND_END_ARG_INFO()
+
+static
 ZEND_BEGIN_ARG_INFO_EX(arginfo_strftime, 0, 0, 1)
        ZEND_ARG_INFO(0, format)
        ZEND_ARG_INFO(0, timestamp)
@@ -156,6 +164,7 @@
        PHP_FE(mktime, arginfo_mktime)
        PHP_FE(gmmktime, arginfo_gmmktime)
        PHP_FE(checkdate, arginfo_checkdate)
+       PHP_FE(checktime, arginfo_checktime)
 
 #ifdef HAVE_STRFTIME
        PHP_FE(strftime, arginfo_strftime)
@@ -1244,7 +1253,6 @@
 }
 /* }}} */
 
-
 /* {{{ proto bool checkdate(int month, int day, int year)
    Returns true(1) if it is a valid date in gregorian calendar */
 PHP_FUNCTION(checkdate)
@@ -1262,6 +1270,29 @@
 }
 /* }}} */
 
+/* {{{ proto bool checktime(int hour, int minute [, int second [, bool natime]])
+   Returns true(1) if it is a valid time */
+PHP_FUNCTION(checktime)
+{
+       long      h, m, s = 0;
+       zend_bool natime = 0;
+
+       if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "ll|lb", &h, &m, &s, &natime) == FAILURE) {
+               RETURN_FALSE;
+       }
+
+       if (h < 0 || h > 23 || m < 0 || m > 59 || s < 0 || s > 59) {
+               RETURN_FALSE;
+       }
+
+       if (natime && (h < 1 || h > 12)) {
+               RETURN_FALSE;
+       }
+
+       RETURN_TRUE;    /* True : The hour, minute, second make a valid time */
+}
+/* }}} */
+
 #ifdef HAVE_STRFTIME
 /* {{{ php_strftime - (gm)strftime helper */
 PHPAPI void php_strftime(INTERNAL_FUNCTION_PARAMETERS, int gmt)
Index: ext/date/php_date.h
===================================================================
RCS file: /repository/php-src/ext/date/php_date.h,v
retrieving revision 1.17.2.11.2.3
diff -u -r1.17.2.11.2.3 php_date.h
--- ext/date/php_date.h 1 Jan 2007 09:35:48 -0000       1.17.2.11.2.3
+++ ext/date/php_date.h 10 Aug 2007 17:04:21 -0000
@@ -36,6 +36,7 @@
 PHP_FUNCTION(gmmktime);
 
 PHP_FUNCTION(checkdate);
+PHP_FUNCTION(checktime);
 
 #ifdef HAVE_STRFTIME
 PHP_FUNCTION(strftime);

This has been open for a long long time, sorry about the wait.

There are too many interpretations of the implementation of this idea that there is no way for us to pick one, and for the function to have configurable behaviour at runtime would be an absolute nightmare.

I'm going to mark as won't fix.