title: Django filter module: lookup expression (year) does not return the desired queryset
body: <p>I am using the <a href="https://django-filter.readthedocs.io/en/stable/index.html" rel="nofollow noreferrer">django-filter module</a>. I would like to get all objects where the year is 2011 or greater.</p>
  <p>this is my setup ...</p>
  <h2>Actual behaviour:</h2>
  <ul>
  <li><p>When I enter a year, then I get the error message: &quot;Enter a valid date.&quot; and the queryset is unchanged.</p>
  </li>
  <li><p>I tried entering a date instead of the year, but then the whole page fails with the error message <code>Field 'None' expected a number but got datetime.date(2011, 1, 1)</code>.</p>
  </li>
  </ul>
  <p><a href="https://i.stack.imgur.com/fDOc1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fDOc1.png" alt="Error Message" /></a></p>
  <h2>The model:</h2>
  <pre><code>class BaseInvoice(models.Model):
      id = ShortUUIDField(primary_key=True)
      invoive_number = models.CharField(max_length=50)
      invoicing_party_ID = models.ForeignKey(InvoicingParty, on_delete=models.CASCADE)
      invoice_type = models.CharField(
          max_length=2, 
          choices=invoice_type,
          default='IN'
          )
      realestate_id = models.ForeignKey(RealEstate, on_delete=models.CASCADE)
      user_group = models.ForeignKey(Group, on_delete=models.CASCADE)
      year_of_contribution = models.DateField(auto_now=False, auto_now_add=False)
      start_of_period = models.DateField(auto_now=False, auto_now_add=False)
      end_of_period = models.DateField(auto_now=False, auto_now_add=False)
      interval = models.ForeignKey(Interval, on_delete=models.CASCADE, default=0)
      currency = models.ForeignKey(Currency, on_delete=models.CASCADE, default='EUR')
      class Meta:
          abstract = True    
      
  
  class RealEstateTax(BaseInvoice):
      realestate_tax = models.DecimalField(max_digits=20, decimal_places=2)
  </code></pre>
  <h2>The Filter</h2>
  <pre><code>import django_filters
  from django_filters import DateFilter
  
  from .models import *
  
  class InvoiceFilter(django_filters.FilterSet):
      
      billing_year = DateFilter(
          label='Abrechnungsjahr',
          field_name='start_of_period', 
          lookup_expr='year__gt'
          )
      
      class Meta:
          model = RealEstateTax
          fields = ['realestate_id', 'billing_year']
  </code></pre>
  <h2>The view</h2>
  <pre><code>from realestate.models import RealEstate
  from django.shortcuts import render
  from django.http import HttpResponse
  from .models import *
  from .filters import InvoiceFilter
  from django.db.models import Sum
  
  def utilities_bill(request):
      realestate_tax = RealEstateTax.objects.all()
      insurance_invoice = InsuranceInvoice.objects.all()
  
      raTaxFilter = InvoiceFilter(request.GET, queryset=realestate_tax)
      inFilter = InvoiceFilter(request.GET, queryset=insurance_invoice)
      realestate_tax = raTaxFilter.qs
      insurance_invoice = inFilter.qs
      realestate_sum = realestate_tax.aggregate(Sum('realestate_tax'))['realestate_tax__sum']
      print(request.GET)
  
      context = {'realestate_tax':realestate_tax, 'insurance_invoice':insurance_invoice, 'raTaxFilter':raTaxFilter, 'realestate_sum':realestate_sum}
      return render(request, 'billing/utilities.html', context)
  </code></pre>
  <h2>This is the first part of the template:</h2>
  <pre><code>{% extends 'main.html' %}
  {% load static %}
  
  {% block title %} Utilities Bill {% endblock %}
  
  {% block content %}
  &lt;img src=&quot;{% static 'media/bojack.png' %}&quot; style=&quot;width:100px&quot;&gt;
  &lt;h1&gt;Nebenkostenabrechnung&lt;/h1&gt;
  &lt;br&gt;
  
  &lt;div class=&quot;container&quot;&gt;
      &lt;div class=&quot;row&quot;&gt;
          &lt;form class=&quot;form-group&quot; method=&quot;get&quot;&gt;
              {{raTaxFilter.form}}
              &lt;button type=&quot;submit&quot; class=&quot;btn btn-primary&quot;&gt;Submit&lt;/button&gt;
          &lt;/form&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  <h2>What I tried so far:</h2>
  <ul>
  <li>Entering an int | --&gt; queryset is unchanged</li>
  <li>Entering a date | --&gt; <code>Field 'None' expected a number but got datetime.date(2011, 1, 1).</code></li>
  <li>tried it with lookup expressions : <code>year</code>, <code>year__gte</code>, <code>year__gt</code> | --&gt; didn't work</li>
  <li>Tested a filter directly in the view <code>realestate_tax = realestate_tax.filter(start_of_period__year__gte=2011)</code> | --&gt; this works but then it is without the <a href="https://github.com/carltongibson/django-filter/blob/78210722d920f803a8142e48969bc37f0f8324ed/docs/index.txt" rel="nofollow noreferrer">django-filter module</a> and I have to build everything by hand</li>
  </ul>
  <p>Many thanks in advance. I hope you can help me out.</p>