TypeError in date_parse.py

[pestrella at gmail.com]

Hello, I'm running an application using python 2.7 with django 1.5 and when posting a page I get the error "TypeError" I guess it is something related to timestamps?
Below I paste part of the code I think is causing the problem and the stacktace I see 

Any help is welcome, Thanks!
Paula

=============================
    # Compute duration for this item.
        duration = None
        if end_timestamp and start_timestamp:
            start_datetime = datetime.fromtimestamp(float(start_timestamp))
            end_datetime = datetime.fromtimestamp(float(end_timestamp))
            duration = end_datetime - start_datetime
=============================
POST
Request URL:	http://127.0.0.1:8000/appraise/evaluation/7b31d05e50584d2b92b4a04d85e68ce3/
Django Version:	1.5.2
Exception Type:	TypeError
Exception Value:	
expected string or buffer
Exception Location:	/usr/local/lib/python2.7/dist-packages/django/utils/dateparse.py in parse_time, line 50
Python Executable:	/usr/bin/python
Python Version:	2.7.2
Python Path:	
['../Appraise-Software/appraise',
 '/home/paula/.local/lib/python2.7/site-packages/setuptools-1.1.4-py2.7.egg',
 '/usr/lib/python2.7/dist-packages',
 '/usr/lib/python2.7',
 '/usr/lib/python2.7/plat-linux2',
 '/usr/lib/python2.7/lib-tk',
 '/usr/lib/python2.7/lib-old',
 '/usr/lib/python2.7/lib-dynload',
 '/home/paula/.local/lib/python2.7/site-packages',
 '/usr/local/lib/python2.7/dist-packages',
 '/usr/lib/python2.7/dist-packages/PIL',
 '/usr/lib/python2.7/dist-packages/gst-0.10',
 '/usr/lib/python2.7/dist-packages/gtk-2.0',
 '/usr/lib/pymodules/python2.7',
 '/usr/lib/python2.7/dist-packages/ubuntu-sso-client',
 '/usr/lib/python2.7/dist-packages/ubuntuone-client',
 '/usr/lib/python2.7/dist-packages/ubuntuone-control-panel',
 '/usr/lib/python2.7/dist-packages/ubuntuone-couch',
 '/usr/lib/python2.7/dist-packages/ubuntuone-installer',
 '/usr/lib/python2.7/dist-packages/ubuntuone-storage-protocol']