I think the second example you are looking at is doing WAY more then is necessary. I would suggest you ignore that one. All you need to get the date difference is this:
int(Date.ToTotalHours(abs(Date.Difference(Date.Parse(Date.ToDateString(Date.Now)), Date.Get(year, month, day, hr, min, sec, mil))) ÷ 24)
The first date, "Date.Parse(Date.ToDateString(Date.Now))" gets the current date without time. That way the time of day won't mess up the difference function.
The second date is where you pass in the birthdate. For the year, use the year of their next upcoming birthday. If their birthday this year has passed, use next year. If not, use this year. For everything after day, you can just pass in 0.
To break it down, the date difference function returns the difference between the 2 dates in milliseconds. Abs so we don't get a negative value. the TotalHours function then gives us the millis in hours so we divide by 24. Finally, we pass it into the int function to just get the integer value representing days.