change MaterialPart.should_be_replaced to .not_too_old

It is more convenient to show the logical negation of should_be_replaced
as booleans are shown as little green or red items by django. Having
a green tick when the item is NOT okay seems a bit counterintuitive.

jdav_web/material/admin.py

@@ -16,7 +16,7 @@ class OwnershipInline(admin.StackedInline):
 class MaterialAdmin(admin.ModelAdmin):
     """Edit view of a MaterialPart"""
     fields = ['name', 'buy_date', 'lifetime']
-    list_display = ('name', 'buy_date', 'lifetime', 'should_be_replaced')
+    list_display = ('name', 'buy_date', 'lifetime', 'not_too_old')
     inlines = [OwnershipInline]
 
 admin.site.register(MaterialPart, MaterialAdmin)

jdav_web/material/models.py

@@ -21,15 +21,15 @@ class MaterialPart(models.Model):
         """String representation"""
         return self.name
 
-    def should_be_replaced(self):
+    def not_too_old(self):
         """Returns wether the part should be replaced cause of age"""
         buy_time = timezone.make_aware(datetime.combine(self.buy_date,
                                                         datetime.min.time()))
-        return yearsago(self.lifetime) >= buy_time
+        return yearsago(self.lifetime) < buy_time
 
-    should_be_replaced.admin_order_field = 'buy_date'
+    not_too_old.admin_order_field = 'buy_date'
-    should_be_replaced.boolean = True
+    not_too_old.boolean = True
-    should_be_replaced.short_description = 'Should be replaced?'
+    not_too_old.short_description = 'Not too old?'
 
 
 class Ownership(models.Model):