time: fix overflow in time.sys_mono_now() on windows

[spytheman]

Checked with running this script on a windows laptop, which failed on master:

import time

wait_times := [
        5 * time.minute + 52 * time.second,
        3 * time.minute + 14 * time.second,
        44 * time.second,
        26 * time.second,
        1 * time.minute + 58 * time.second,
        4 * time.second,
        4 * time.minute + 27 * time.second,
        3 * time.minute + 24 * time.second,
        5 * time.minute + 3 * time.second,
        1 * time.minute + 56 * time.second,
        1 * time.minute + 8 * time.second,
        12 * time.second,
        10 * time.minute + 8 * time.second,
]

mut wt := 0
mut sw := time.new_stopwatch()
for wt != wait_times.len {
        println('wt: ${wt} | wait_times.len: ${wait_times.len} | sleeping for: ${wait_times[wt]} | SW: ${sw}')
        time.sleep(wait_times[wt])
        sw.stop()
        if sw.end < sw.start {
                println('Negative: ${sw}')
                println('>>>> wt was: ${wt}')
                break
        }
        println('Elapsed: ${sw.elapsed()}')
        sw.start()
        wt++
}
println('Success, wt: ${wt}')

On this PR, it produced:

#0 14:14:24 ᛋ fix_overflow_in__time_sys_mono_now__on_windows /d/programs/v❱v run /d/programs/misc/2026_01_06__14/x.v
wt: 0 | wait_times.len: 13 | sleeping for: 352000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 6500
    end: 0
}
Elapsed: 5:52.003
wt: 1 | wait_times.len: 13 | sleeping for: 194000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 352003608500
    end: 0
}
Elapsed: 3:14.007
wt: 2 | wait_times.len: 13 | sleeping for: 44000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 546011778000
    end: 0
}
Elapsed: 44.005s
wt: 3 | wait_times.len: 13 | sleeping for: 26000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 590017421300
    end: 0
}
Elapsed: 26.004s
wt: 4 | wait_times.len: 13 | sleeping for: 118000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 616021871000
    end: 0
}
Elapsed: 1:58.003
wt: 5 | wait_times.len: 13 | sleeping for: 4000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 734025659500
    end: 0
}
Elapsed: 4.015s
wt: 6 | wait_times.len: 13 | sleeping for: 267000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 738040947600
    end: 0
}
Elapsed: 4:27.003
wt: 7 | wait_times.len: 13 | sleeping for: 204000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 1005044610600
    end: 0
}
Elapsed: 3:24.006
wt: 8 | wait_times.len: 13 | sleeping for: 303000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 1209051341600
    end: 0
}
Elapsed: 5:03.011
wt: 9 | wait_times.len: 13 | sleeping for: 116000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 1512063166200
    end: 0
}
Elapsed: 1:56.012
wt: 10 | wait_times.len: 13 | sleeping for: 68000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 1628076211100
    end: 0
}
Elapsed: 1:08.002
wt: 11 | wait_times.len: 13 | sleeping for: 12000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 1696079343500
    end: 0
}
Elapsed: 12.006s
wt: 12 | wait_times.len: 13 | sleeping for: 608000000000 | SW: time.StopWatch{
    elapsed: 0
    start: 1708086601900
    end: 0
}
Elapsed: 10:08.007
Success, wt: 13
#0 14:53:08 ᛋ fix_overflow_in__time_sys_mono_now__on_windows /d/programs/v❱

which is consistent with the output on a macos and linux machines.

Fixes: 26265

TLDR: avoid overflow produced by multiplying by 1000000000, just to delete after that by 10000000 . In essence, that is now replaced by multiplying by 100 (for the windows machines we tested on).

[spytheman]

Drawbacks: with this approach, a potential overflow could happen, for a stopwatch running for more than 5.8 years:

0[15:04:53]@m1: (master) /opt/v $ bc
>>> scale=16
>>> ((2^64 / 100 ) / 1000000000) / (3600*24*365)
5.8494241735507203
>>>

however I think it is an acceptable compromise (the same can happen on macos, that uses a conversion by multiplying/dividing too), and stopwatches are used for measuring a lot shorter durations normally.

[spytheman]

(the CI failures are not related)