lastModifiedTime doesn't seem to be working - python example

lastModifiedTime doesn't seem to be working - python example

I'm trying to get records for a lastModifiedTime greater than a particular date but it's giving me records that don't match the date criteria.  Any help or clarification would be appreciated:

authtoken = 'my_auth_token'
searchCondition="lastModifiedTime=2015-01-01"
params = {'authtoken':authtoken,'scope':'crmapi','id':id,'selectColumns':'All','searchCondition':searchCondition  }
final_URL = " https://crm.zoho.com/crm/private/xml/Accounts/getMyRecords"
data = urllib.urlencode(params)
request = urllib2.Request(final_URL,data)
response = urllib2.urlopen(request)
xml_response = response.read()