T.G.I.F vs. S.H.I.T

A business man got on an elevator. When he entered, there was a blonde already inside who greeted him with a bright, "T-G-I-F."
He smiled at her and replied, "S-H-I-T."
She looked puzzled and repeated, "T-G-I-F," more slowly.
He again answered, "S-H-I-T."
The blonde was trying to keep it friendly, so she smiled her biggest smile, and said as sweetly as possibly, "T-G-I-F."
The man smiled back to her and once again, "S-H-I-T."
The exasperated blonde finally decided to explain.
'T-G-I-F' means 'Thank Goodness It's Friday.' Get it, duuhhh?"
The man answered, "'S-H-I-T' means 'Sorry, Honey, It's Thursday.'"