You say "Guten Morgen" in the morning. (Guten Morgen = Good Morning). and "Guten Tag" is said in the afternoon. (Guten Tag = Good afternoon)

"Guten Tag" is the normal way of greeting a person when you meet her. It is completely independent of the time of the day. It is used especially in the northern part of the German-speaking area wheras in the southern part "Grüß Gott" is more common. "Guten Morgen" however may be used only in the morning (until 10am at most I would say). When used later in the day "Guten Morgen" could be understood in an ironical sense (implying that the person you are talking to is not yet awake). So be carefull! In the evening you may say "Guten Abend" when you meet someone and "Gute Nacht" to say goodbye. You do NOT say "Guten Nachmittag" in German! In earlier times "guten Tag" was also used to say goodbye and especially to tell a person that you want to put an end to the conversation. This is obsolete today but you may find it in literature. Of course today there are also a lot of other forms of greating like "Hallo", "Hi", "Ciao", "Servus" and so on.

Guten morgen 6 Uhr bis 11 Uhr Guten Tag 11 Uhr bis 18 Uhr

Basically Ismail is right. To elaborate a little: Guten Morgen - only in the morning! Guten Tag - Can be used any time of the day. Guten Abend - only in the evening Gute Nacht - Only night time, and only as a 'bye bye' when you leave. Exception: The West Coast, there you can use 'Moin' or 'Moin Moin' 24 hours a day.

no only in the afternoon @ismail. I would say it starts in the late morning (~11:30am) until 6-7pm.