Python3 ImportError: No module named 'dateclass'

Question

I have a simple module with this file structure:

main.py
/mypackage
    __init__.py
    dateclass.py
    myfunctions.py

Here are the imports and relevant classes/functions of each file. I suspected a circular dependency, but don't see one.

main.py:

from mypackage.myfunctions import testfunction

dateclass.py:

#!/usr/bin/env python3
import datetime

class Date(object):
    ...

myfunctions.py:

#!/usr/bin/env python3
from dateclass import date

def testfunction():
    print("You imported me!")

When I run "python3 main.py" it returns:

myfunctions.py", line 2, in <module>
from dateclass import Date
ImportError: No module named 'dateclass'

Am I missing something that's staring me in the face here? Much appreciate the help.