## Bug description

The new funnels time conversion endpoint sometimes returns bins with negative time values. This makes it difficult to reflect accurate time conversion funnels data on the frontend. @Twixes @neilkakkar 

<img width="379" alt="Screen Shot 2021-07-13 at 9 14 42 PM" src="https://user-images.githubusercontent.com/13460330/125560350-3ef2eb03-5fc5-4c89-a0ab-988c945d7d87.png">

Example CH localhost [link ](http://localhost:8000/insights?insight=FUNNELS&properties=%5B%5D&filter_test_accounts=false&interval=day&events=%5B%7B%22id%22%3A%22%24pageview%22%2C%22name%22%3A%22%24pageview%22%2C%22type%22%3A%22events%22%2C%22order%22%3A0%7D%2C%7B%22id%22%3A%22%24pageview%22%2C%22name%22%3A%22%24pageview%22%2C%22type%22%3A%22events%22%2C%22order%22%3A1%7D%2C%7B%22id%22%3A%22%24pageview%22%2C%22name%22%3A%22%24pageview%22%2C%22type%22%3A%22events%22%2C%22order%22%3A2%7D%5D&display=FunnelsTimeToConvert&actions=%5B%5D&new_entity=%5B%5D) to reproduce bug.

Request Payload

<img width="612" alt="Screen Shot 2021-07-13 at 9 19 06 PM" src="https://user-images.githubusercontent.com/13460330/125560592-9e2a0670-606c-485a-a102-7e085c72f225.png">


## Expected behavior

No negative time bin values. 

## How to reproduce

1. Navigate to new funnels time conversion page. Make sure to turn on this FF: `4535-funnel-bar-viz`
2. Add a few identical actions/events
3. Open up network requests to see the response to `api/funnels`

## Environment

- [X] PostHog Cloud
- [ ] self-hosted PostHog, version/commit: _please provide_

#### *Thank you* for your bug report – we love squashing them!

ee/clickhouse/queries/funnels/test/test_funnel_time_to_convert.py
from uuid import uuid4
from ee.clickhouse.models.event import create_event
from ee.clickhouse.queries.funnels import ClickhouseFunnel, ClickhouseFunnelStrict, ClickhouseFunnelUnordered
from ee.clickhouse.queries.funnels.funnel_time_to_convert import ClickhouseFunnelTimeToConvert
from ee.clickhouse.util import ClickhouseTestMixin
from posthog.constants import INSIGHT_FUNNELS, TRENDS_LINEAR
from posthog.models.filters import Filter
from posthog.models.person import Person
from posthog.test.base import APIBaseTest
FORMAT_TIME = "%Y-%m-%d %H:%M:%S"
FORMAT_TIME_DAY_END = "%Y-%m-%d 23:59:59"
def _create_person(**kwargs):
    person = Person.objects.create(**kwargs)
    return Person(id=person.uuid, uuid=person.uuid)
def _create_event(**kwargs):
    kwargs.update({"event_uuid": uuid4()})
    create_event(**kwargs)
class TestFunnelTrends(ClickhouseTestMixin, APIBaseTest):