Python: How do I include a function in another function?

I'm just starting to study programming and I have a question about functions. I defined the function leapyear as follows:

def leapyear(n):
  # Given a year, decide if it is a leap year or not.
  if n%4==0:
    if n%100==0:
      if n%400==0:
        return "It is a leap year."
      else:
        return "It is not a leap year"
    else:
      return "It is a leap year"
  else:
    return "It is not a leap year"

Now I want to use it in another function, which is going to tell me if I wrote a valid date or not (for example, 02/28/2021 is valid, but 02/29/2021 is not). What I did is this:

def valid_date():
  day = int(input("Day: "))
  month = input("Month: ")
  year = int(input("Year: "))
  
  if month=="January":
    if day<=31:
      print("Valid date")
    else:
      print("Not a valid date")

  if month=="February":
    if year%4==0:
      if year%100==0:
        if year%400==0:
          if day<=29:
            print("Valid date")
          else:
            print("Not a valid date")
        else:
          if day<=28:
            print("Valid date")
          else:
            print("Not a valid date")
      else:
        if day<=29:
          print("Valid date")
        else:
          print("Not a valid date")
    else:
      if day<=28:
        print("Fecha válida")
      else:
        print("Fecha no válida")

  if month=="March":
    if day<=31:
      print("Valid date")
    else:
      print("Not a valid date")

It actually gets to December, but it doesn't really matter because it is basically the same.

The code works, but I was thinking to use the leapyear function instead of writing the whole code again. How can I do that?

Thanks!