Function to validate date format
This is already pretty simple. But I'd use the ternary operator ?: to make it a bit more compact:

function validateDateFormat(format) { 
    return format.indexOf("yyyy") == -1 ? format : format.replace(/yy/g, 'y'); 
}


Also, perhaps you can be a bit more accurate.
There is a discrepancy between your check and the action taken:


Check if string contains "yyyy"
Replace all occurrences of "yy" with "y"


Perhaps it would be more accurate to change the action to this:


  Replace "yyyy" with "yy" (only once)


That is:

function validateDateFormat(format) { 
    return format.indexOf("yyyy") == -1 ? format : format.replace(/yyyy/, 'yy'); 
}


Actually, and probably I should have started with this:
since the .replace method returns the original string if there is no match,
you could simplify even further:

function validateDateFormat(format) { 
    return format.replace(/yyyy/, 'yy'); 
}