Hey Jessie. They are not the same. Guten Tag = good day While Guten Nachmittag = Good afternoon Tag = dag Nachmittag = afternoon where, mittag = mid-day (noon) and, nach = after After mid-day, also known as after-noon.^^ Hope that explains it.

Wik already explained the different literal meanings, but it should also be mentioned that "guten Nachmittag" normally isn't used in German. Between "guten Morgen" and "guten Abend" there's just "guten Tag", with no further distinction.