time module: Week of Year is one day too late

[Brixy]

Describe the bug

Hi guys,

Maybe I found an error in the time module: The calculation of the "Week of Year" seems to be one day too late.

The 10th calendar week starts next Monday, 2025-03-03. Yet, ww for that date still shows week "09".

The 10th week falsely starts on Tuesday, 2025-03-04.

Date calculation is a "horrible" topic. So please bear with me if I have made a mistake ;-)

Reproduction Steps

import time

s := '2025-03-03 12:48:34'
t := time.parse(s)!
// `ww` should print `10`; instead it prints `09`
println(t.custom_format('YYYY-MM-DD ww (ddd)'))
// Apparently, a "Week of Year" starts on Tuesday (instead of Mondays)
// `ww` for 2025-03-04 is `10`
println(t.add_days(1).custom_format('YYYY-MM-DD ww (ddd)'))

Expected Behavior

ww should be "10" for both dates in the script.

Current Behavior

The first date (2025-03-03) prints 09 for ww.

Possible Solution

No response

Additional Information/Context

Shell programs handle this correctly. So I guess it's a V issue.

V version

0.4.9 f83af8a

Environment details (OS name and version, etc.)

Chimera Linux
Linux 6.12.16-0-generic
Wayland

(also tested on Void Linux)

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[huly-for-github]

Connected to Huly®: V_0.6-22240

[jorgeluismireles]

Golang gives week=10 (as you said it should):

package main

import (
	"fmt"
	"time"
)

func main() {
	t := time.Date(2025, 3, 3, 12, 00, 00, 0, time.UTC)
	year, week := t.ISOWeek()
	fmt.Printf("year: %v\n", year)
	fmt.Printf("week: %d\n", week)
}

year: 2025
week: 10

[spytheman]

With latest V, thanks to @kbkpbot's PR:

#0 10:10:53 ^ master ~/code/v>cat z.v
import time

s := '2025-03-03 12:48:34'
t := time.parse(s)!
// `ww` should print `10`; instead it prints `09`
println(t.custom_format('YYYY-MM-DD ww (ddd)'))
// Apparently, a "Week of Year" starts on Tuesday (instead of Mondays)
// `ww` for 2025-03-04 is `10`
println(t.add_days(1).custom_format('YYYY-MM-DD ww (ddd)'))

#0 10:11:00 ^ master ~/code/v>v run z.v
2025-03-03 10 (Mon)
2025-03-04 10 (Tue)
#0 10:11:03 ^ master ~/code/v>