Before I comment further, I want to write about the various greetings used by Japanese that I know of at this juncture (I am sure there are many more that I do not know yet!!):

Before you eat, you will say 'itadakimasu'
After you eat, you will say 'gochisousamadeshita'

Before you go out of house, you will say 'ittekimasu'
After you come home, you will say 'tadaima'

For person at home, you will say 'itterashai' to person leaving
For person at home, you will say 'okaerinasai' to person who just came home

When going to bed, you will say 'oyasuminasai'