dates = [{'created': 1}, {'created': 2}, {'created': 3}, {'created': 4}]
elts = [{'created': 2, 'key1': 'val1'}, {'created': 4, 'key1': 'val2'}]

all_dates = set(e['created'] for e in dates) # gets a list of all dates that exist in `dates`
elts_dates = set(e['created'] for e in elts) # same for elts

print "All dates:", all_dates
print "Elts dates:", elts_dates

missing_dates = all_dates - elts_dates

print "Missing dates:", missing_dates

for entry in missing_dates:
    elts.append(dict(created=entry))

print elts

All dates: set([1, 2, 3, 4])
Elts dates: set([2, 4])
Missing dates: set([1, 3])
[{'key1': 'val1', 'created': 2}, {'key1': 'val2', 'created': 4}, {'created': 1}, {'created': 3}]