Meaning of gǎi tiān:
改天
another day; some other time
Wǒ jīntiān yǒu shì, gǎitiān zài jiànmiàn ba.
我今天有事,改天再见面吧。
I have something to do today, let's meet another day.
wǒmen gǎi tiān zài jiàn miàn ba.
我们改天再见面吧。
Let's meet some other time.
to find another day (for appointment etc); to take a rain check
Wǒmen gǎitiān zài jiànmiàn ba, jīntiān wǒ yǒudiǎn máng.
我们改天再见面吧,今天我有点忙。
Let's meet another day, I'm a bit busy today.
Wǒmen jīntiān tài máng le, gǎitiān zài jiànmiàn ba.
我们今天太忙了,改天再见面吧。
We are too busy today, let's take a rain check on meeting up.