var today = new Date();
if((today.getHours() >=4) && (today.getHours() <12)) { document.write("Guten Morgen") }
if((today.getHours() >=12) && (today.getHours() <17)) { document.write("Guten Tag") }
if((today.getHours() >=17) && (today.getHours() <24) || (today.getHours() >=0) && (today.getHours() <4)) { document.write("Guten Abend") }