야 + 자 + time

In this case, '야' would say that 'Hey'.
More, it is the one of name for calling someone who would be same age or younger than one person.
But we shouldn't use it any case because it is informal.
And sometimes this word have a meaning that someone pay no mind to one another.
So we should use it between 2 or more persons who are very close friends and known each other for many times.
And it could be often used to start to say something adding few words.
For example,
'Hey, cheol-soo.' : 야, 철수야.
'Hey, as you know' : 야, 있잖아.. or 야, 너도 알다시피.. or 야, 근데..

'자' means (in this case, '~자.) 'Let's ~'
'Let's go out' : '나가자~', it doesn't mean 나갑시다~ (높임말)
'Let's have a dinner' : '저녁먹자~', it isn't also 저녁 먹읍시다~ or 식사 합시다~ (높임말)

And then, here is a word of idiomatic expression and common using, not formal, '야자하다'.
It means 'to speak impolitely each other'
For example,
'Cheol-su and I, we speak impolitely each other'
: 철수와 나는 서로 '야, 자~' 하는 (반말하는) 사이야'