Expressing “on time/in time” with 按时 and 及时.
These two words are translated the same way but slightly differ in meaning.
The adverb 按时
We use the adverb 按时 
to emphasize that an action occurs at a designated time. For example, “to get up on time”.
We should state it before the predicate in a sentence.
to emphasize that an action occurs at a designated time. For example, “to get up on time”.
We should state it before the predicate in a sentence.
Examples
你要按时吃药
You need to take your medicine on time
nǐ yào ànshí chī yào
总有几个同学不能按时完成作业
There are always a few students who can't finish their homework on time
zǒng yǒu jǐ ge tóngxué bù néng ànshí wánchéng zuòyè
The adjective 及时
We use the adjective 及时 to indicate that the action happened just before a designated time. For example, “the doctor arrived just in time”.
In this case, it is often followed by the particle 的 .
to indicate that the action happened just before a designated time. For example, “the doctor arrived just in time”.
In this case, it is often followed by the particle 的 .Examples
你来得很及时
You came just in time
nǐ lái de hěn jíshí
谢谢你给我及时的帮忙
Thank you for your timely help
xièxie nǐ gěi wǒ jíshí de bāngmáng
We can use 及时 as an adverb with the meaning of “immediately”.
Examples
有问题要及时问老师
If there are any questions, you should immediately ask the teacher
yǒu wèntí yào jíshí wèn lǎoshī
及时关好窗户
Close the windows immediately
jíshí guānhao chuānghu