text: Title: Unexpected "down" after sending ping
  Body: I have a test check setup on healthchecks.io, configured with
  
  Cron Expression | `* 9 * * *`
  -- | --
  Time Zone | America/Los_Angeles
  Grace Time | 30 minutes
  
  This triggers at 9:30AM local time (as expected), and I send a ping to put it back the "up" state. ~30 minutes after the ping, the check goes back to "down".
  
  Here's a screenshot of the details page, with the unexpected transitions highlighted:
  ![image](https://github.com/healthchecks/healthchecks/assets/6877802/49526580-53ce-4a06-b947-e1e4cda60a0c)
  
  Chronologically (with my comments):
  ```
  May 21 | 09:30 | Status: up ➔ down.                                     # expected, that's 30 minutes after the cron time.
  May 21 | 09:34 | OK | HTTPS POST from x.x.x.x - python-requests/2.31.0  # manual ping
  May 21 | 09:34 | Status: down ➔ up.                                     # expected after ping
  May 21 | 10:05 | Status: up ➔ down.                                     # unexpected! suspiciously at "grace time" after the last ping.
  May 21 | 10:21 | OK | HTTPS POST from x.x.x.x - python-requests/2.31.0  # manual ping to shut it up
  May 21 | 10:21 | Status: down ➔ up.                                     # expected after ping
  ```
  
  ```
  May 22 | 09:30 | Status: up ➔ down.                              # expected, that's 30 minutes after the cron time.
  May 22 | 09:41 | OK | HTTPS POST from x.x.x.x - Mozilla/5.0 ...  # manual ping from UI
  May 22 | 09:41 | Status: down ➔ up.                              # expected after ping
  May 22 | 10:12 | Status: up ➔ down.                              # unexpected! 
  May 22 | 10:13 | OK | HTTPS POST from x.x.x.x - Mozilla/5.0 …    # manual ping to shut it up
  May 22 | 10:13 | Status: down ➔ up.                              # expected after ping
  ```
  
  Is my expectation of how this should work incorrect? Could there be something funny going on due to the non-UTC timezone?