Debugging a Todo App

So lately I’ve been boasting about how easy it is to build Progressive Web Apps. I’ve been dogfooding my glorified todo app Mute Swan for weeks thinking it was perfect (bug-free) and quite possibly a game changer, if I dare say.

As it turns out these things are not so easy to test. This week I squashed a bug that I may never have noticed.

Daylight Savings

Earlier in the week I noticed that tasks assigned to Monday, 30th March would also appear under Sunday 29th. Here’s a GIF:

Mute Swan daylight savings bug

I had my suspicions. I Googled “when do the clocks change” as is tradition. 29th March is guilty this time of year in the UK. Now I have a clue as to why this bug exists; time to debug.

Mute Swan tasks are stored by ID in a single state object. Each task has a unix timestamp. This time is set to midnight for the day they’re assigned. When the list for each day is rendered the state is filtered by a 24 hour range:

const tasks = Object.values(state)
  .filter(task =>
    task.unix >= startTime && task.unix < endTime

Using “today” as an example, I start the range at midnight:

const startDate = new Date();
startDate.setHours(0, 0, 0, 0);

Then add 24 hours to get the end of the range:

const endDate = new Date(
  startDate.getTime() + 86400000

That seemed to work fine. Until the clocks went forward. With this code the endDate for the range on Sunday 29th is one hour past midnight on the 30th. That means any tasks assigned to Monday also appear in the filtered list for Sunday.


Now given the Redux nature of my code, if I dragged either of those duplicate tasks into a different day, I’d no longer see two because only the timestamp for a single task ever changed in the state. This was a bug in the selector logic.

The solution:

const endDate = new Date(startDate.getTime());
endDate.setDate(endDate.getDate() + 1);

This technically does the same thing by adding 24 hours to the date. However, when I do endDate.getTime() it will return 3600000 milliseconds (one hour) less than before. JavaScript Date methods account for daylight savings whereas my naively adding unix time together did not.

Lesson learned…

Now that I think about it I remember throwing together an “event countdown” for a client that ended up being off by an hour. I’ve made this mistake before! Thankfully we got that fixed long before attendees arrived on the day.

I’m blogging this so I don’t forget again.

Last updated: June 2020.

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