function checktime(thetime) {
var a,b,c,f,err=0;
var a=thetime.value;
if (a.length != 5) err=1;
b = a.substring(0, 2);
c = a.substring(2, 3);
f = a.substring(3, 5);
if (/\D/g.test(b)) err=1; //not a number
if (/\D/g.test(f)) err=1;
if (b<0 || b>23) err=1;
if (f<0 || f>59) err=1;
if (c != ':') err=1;
if (err==1) {
alert ('Die eingegebene Zeit ist ungueltig.\nGeben Sie die Zeit in folgendem Format ein XX:XX ');
thetime.value = "00:00";
thetime.focus();
}
}

function checktimecomplete(thetime) {
var a=thetime.value;
if (a.length == 2) {
thetime.value = thetime.value + ':';
}
}