Error: Whenever I am trying to download the excel sheet. I am getting an error saying Type Error.

[avikmrr]

LINK: http://127.0.0.1:8000/weather/history/

TypeError at /weather/history/
unsupported operand type(s) for -: 'NoneType' and 'NoneType'
Request Method: POST
Request URL: http://127.0.0.1:8000/weather/history/
Django Version: 1.8
Exception Type: TypeError
Exception Value:
unsupported operand type(s) for -: 'NoneType' and 'NoneType'
Exception Location: /Users/Desktop/virtualenv/lib/python2.7/site-packages/xlsxwriter/worksheet.py in _isinf, line 4517
Python Executable: /Users/avinash.af.kumar/Desktop/virtualenv/bin/python
Python Version: 2.7.10
Python Path:
['/Users/Desktop/python-django-exporting-files-master',
'/Users/Desktop/virtualenv/lib/python27.zip',
'/Users/Desktop/virtualenv/lib/python2.7',
'/Users/Desktop/virtualenv/lib/python2.7/plat-darwin',
'/Users/Desktop/virtualenv/lib/python2.7/plat-mac',
'/Users/Desktop/virtualenv/lib/python2.7/plat-mac/lib-scriptpackages',
'/Users/Desktop/virtualenv/lib/python2.7/lib-tk',
'/Users/Desktop/virtualenv/lib/python2.7/lib-old',
'/Users/Desktop/virtualenv/lib/python2.7/lib-dynload',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-darwin',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/lib-tk',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-mac',
'/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/plat-mac/lib-scriptpackages',
'/Users/Desktop/virtualenv/lib/python2.7/site-packages']
Server time: Thu, 20 Oct 2016 07:04:07 +0000

Please help me fix this error. Thanks in advance.