TL;DR:
new Date('2000-01-01') creates a date at midnight UTC, not local time.
In Japan (UTC+9), that means 2000-01-01T09:00:00, so our dashboard filters missed all data created before 9AM.
The Bug
We noticed something strange while fetching data for an admin dashboard: records created before 9AM were missing!
Our date filters were simple:
<input type="date" />
Which outputs a simple string like
const minDateString = '2000-01-01';
So we wrapped it in a Date constructor and compared it in our filter
new Date(minDate) < item.date && item.date < new Date(maxDate);
How hard can it be, right? Well...
Why It Happened
I knew that new Date('YYYY-MM-DD') sets the time to midnight. What I didn’t know was that it sets it to midnight in UTC.
For Japan (UTC+9), that means the Date actually becomes 2000-01-01T09:00:00+09:00.
So when we said “greater than January 1st”, we really meant “greater than 9AM on January 1st”. Hence, anything created from midnight to 8:59:59 was excluded.
The Fix
Instead of letting new Date() assume UTC, explicitly set it to midnight. (And while we're at it, set the max date range to 1ms before the next day.)
const minDate = new Date(`${minInputDate}T00:00:00`);
const maxDate = new Date(`${maxInputDate}T23:59:59.999`);
Lessons Learned
new Date('YYYY-MM-DD')→ midnight UTC- Always check how date strings are interpreted when time zones matter.
- For filters, prefer constructing with year/month/day explicitly or append a local time.