Format a decimal with both leading and trailing 0's

Question

Both ChatGPT & Copilot have failed me. I am seeking to add padding to a time that gives me two digits for whole seconds and three digits for fractional seconds. So, given $time which is produced by

$timer = [Stopwatch]::StartNew()
$timer.Stop()
$time = $timer.Elapsed.TotalSeconds

AI suggests things like

$time = [Math]::Round($time, 3)
$time = "{0:000.000}" -f $time

When I try to use the resulting time like this

"$($time): $function"

I get a single 0 to the left of the decimal, and no trailing zeros added.

Where am I (and AI) going wrong?

"a time that gives me two digits for whole seconds" - what behavior are you hoping for if the TotalSeconds value exceeds 100.0? — Mathias R. Jessen
– Mathias R. Jessen, Commented Apr 2 at 13:20
I don't expect it to exceed 99 seconds, and if it does once in a while I'll just live with it, or push to three digits. But I am just using it to profile performance of some functions as I refactor some things, so it's not super important. But constantly getting one or two times with only two digits when the vast majority have had three, is making a quick understanding of the numbers problematic. — Gordon
– Gordon, Commented Apr 2 at 13:25
FWIW "{0:00.000}" -f $time is the correct solution (you can skip Math.Round, formatter will take care of it for you) - perhaps you've typed $time as a [decimal] or [double] somewhere earlier in the script? Another explanation could be that because you stop the timer almost immediately, the elapsed time is <5ms, which would indeed result in 0.000. Does it work as expected if you sleep + use a new variable? — Mathias R. Jessen
– Mathias R. Jessen, Commented Apr 2 at 13:28
$time is being passed as a [Double], because ($timer.Elapsed.TotalSeconds).GetType().Fullname) said that's what the timer was providing. Do I need to work from a [DateTime] instead? Or is there another approach I need when formatting a double? — Gordon
– Gordon, Commented Apr 2 at 13:34
Okay there's your explanation - assign the output of '{0:00.000}' -f $time to some variable that isn't typed [double] (appending to a [List[string]] or [List[psobject]] would work) — Mathias R. Jessen
– Mathias R. Jessen, Commented Apr 2 at 13:50

Mathias R. Jessen · Accepted Answer · 2025-04-02 14:03:30Z

3

Using the format string 00.000 is indeed correct, but since you're assigning the resulting string back to $time (which appears to have a [double] type attribute attached) it gets parsed and converted back to a [double] immediately, which is why it suddenly renders as just 0 when you later use it in an expandable string.

Assign the output from the -f operation to a variable that isn't [double]-typed and it'll work just fine:

$timer = [Stopwatch]::StartNew()
$timer.Stop()
[double]$time = $timer.Elapsed.TotalSeconds

$timestamp = '{0:00.000}' -f $time

"${timestamp}: ... is how long it took"

answered Apr 2 at 14:03

Mathias R. Jessen

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