Parse, format and sort dates
Two silent failures: the impossible date 2025-03-32 becomes 1 April, and the default sort puts 30 March first.
A timestamp is one exact instant, but the calendar day it falls on depends on whose clock you read. Parse dates with a stated format, format them on a stated clock, and never let a chart silently shift a ride to the wrong day.
Change the code and press Run (Ctrl+Enter). Predict what will happen first, then break it on purpose and read what changes. Your edits are saved and match the lesson page.
Ready for more? This phase has 5 hands-on exercises with automatic checks.
Practice questions
Write the code in the editor, run it, then open the model answer to compare.
Write localDay(iso, zone) returning YYYY-MM-DD. Use it to count how many of trips-10k.csv's trips start on 15 June on the Pacific/Auckland clock.
Write hoursInLocalDay(dateText, zone) that returns 23, 24 or 25 for a YYYY-MM-DD date by counting the hours.
Explain it without notes
Why can a trip at 00:30 UTC belong to the previous day for some viewers?
How do you get the local day of an instant for Ardenvale without a library?
Why is dividing a daily total by 24 wrong on 30 March 2025?
Console
(nothing was printed)Expected console output
utcParse('13/03/2025'): 2025-03-13T00:00:00.000Z
utcParse('2025-03-32'): 2025-04-01T00:00:00.000Z
utcParse('2025-03-12 '): null
new Date('13/03/2025'): Invalid Date
utcFormat: Wednesday 18 June 2025, 08:15
Ardenvale: Wednesday, 18 June 2025 at 08:15
Auckland: Wednesday, 18 June 2025 at 20:15
default sort() : 2025-03-30 2025-03-04 2025-03-12
d3.ascending : 2025-03-04 2025-03-12 2025-03-30
ISO text sorts too: 2025-03-04 2025-03-12 2025-03-30
but this text does not: 04/03/2025 13/03/2025 30/01/2025