Example sentences
jīn今tiān天xīng星qī期jǐ几？
What day is today?
wǒ我jīn今tiān天bù不shàng上bān班。
I'm not working today.
About this word
今 means "this, present" and 天 means "day", so 今天 is literally "the present day", which is today. The pattern is the real lesson: 昨 slots in for the past and 明 for what is coming, giving 昨天, 今天, 明天, and 天 keeps working in 前天 and 后天. 今 also builds 今年 (this year), 今晚 (tonight) and 今后 (from now on). Because these words are all relational, 明天 always means the day after whatever day you happen to be speaking on. The same 天 sits in 春天 and 夏天, where it names a season instead of a day.
The story behind the character
今
今 is a lid-like top (亼) with a stroke under it, and nobody has settled what it first drew. Xu Shen's whole entry is 今，是時也, now is this moment; he defined the word and left the picture alone. Scholars have since guessed a wooden bell, an upside-down mouth, a roof. What is certain: oracle bones already use it for today, as in 今夕其雨, will it rain tonight.
天
大 is a person with arms spread. Add one stroke on top and you get 天. The space above a person's head. So it didn't start as "sky," it started as "the top of your head." Only later did it climb up to mean heaven.