Yes, the difference between "Guten Tag" and "Guten Morgen" is precisely as you described. You can basically use "Guten Tag" (good day) at all times of the day, while you use "Guten Morgen" (good morning) only in the morning. There is also "Guten Abend" (good evening) which you would use only in the evening. These phrases are all used if you wish to greet somebody. Please note that you would never use "Gute Nacht" (good night) to greet somebody. You only use the latter to say goodbye. All in all this seems to be the same as in English. Note however that in Germany nobody would say "Guten Nachmittag" (good afternoon).

In my book it says "guten tag" it's used at 6 p.m.