Date validation

Hi everyone!
I've always been curious about programming so I'm taking a class as a hobby but seeing as I'm a total beginner I'm pretty lost.

I have a tuple where some of the variables are a dates,:
insert

Code:

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typedef struct {
    tDate packingDate;
    tDate bestBeforeDate;
} tProduct;


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And then in a different c file I've been given this
insert

Code:

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void dateInit(tDate *date, int day, int month, int year){
    date->day = day;
    date->month = month;
    date->year = year;
}

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And a bool validateDate(tDate d) where it checks that the dates are correct. I need to validate the dates but I don't understand what's the use of dateInit. Also, if these functions are in a different file, how should I use them inside my function?
Do you know any book of webpage for beginners that could be helpful? I'm seeing I really need it and I'd be really grateful! Thank you!

Something like this

Code:

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tProduct milk;
dateInit(&milk.bestBeforeDate, 25, 12, 2020 );

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