Invoke-RestMethod, when given a JSON response, automatically parses it into a [pscustomobject] graph; in a manner of speaking, it has ConvertFrom-Json built in.
When ConvertFrom-Json does recognize what are invariably string representation of dates in the input JSON, it converts them to [datetime] instances.
In Windows PowerShell (v5.1, the latest and final version) and as of PowerShell (Core) 7.2, you get NO control over what kind of [datetime] instances are constructed, as reflected in their .Kind property:
In Windows PowerShell, which requires a custom date-string format (e.g. "\/Date(1633984531266)\/"), you invariably get Utc instances.
In PowerShell (Core) 7+, which additionally recognizes string values that are (variations of) ISO 8601 date-time strings (e.g. "2021-10-11T13:27:12.3318432-04:00"), the .Kind value depends on the specifics of the string value:
- If the string ends in
Z, denoting UTC, you get a Utc instance.
- If the string ends in a UTC offset, e.g.
-04:00 you get a Local instance (even if the offset value is 00:00)
- Note that this means that the timestamp is translated to the caller's local time zone, so the original offset information is lost (unless the caller's time zone's offset happens to match).
- Otherwise you get an
Unspecified instance.
While Windows PowerShell will see no new features, there is a hope for PowerShell (Core): GitHub issue #13598 proposes adding a -DateTimeKind parameter to ConvertFrom-Json, so as to allow explicitly requesting the kind of interest, and to alternatively construct [datetimeoffset] instances, which are preferable.
Workaround:
- Note: In the event that you need access to the raw string values, exactly as defined, the solution below wont' work. You'll have to retrieve the raw JSON text and perform your own parsing, using
Invoke-WebRequest and the response's .Content property, as Mathias R. Jessen notes.
The following snippet walks a [pscustomobject] graph, as returned from Invoke-RestMethod and explicitly converts any [datetime] instances encountered to Local instances in place (Unspecified instances are treated as Local):
# Call Invoke-RestMethod to retrieve and parse a web service's JSON response.
$fromJson = Invoke-RestMethod ...
# Convert any [datetime] instances in the object graph that aren't already
# local dates (whose .Kind value isn't already 'Local') to local ones.
& {
# Helper script block that walks the object graph.
$sb = {
foreach ($el in $args[0]) { # iterate over elements (if an array)
foreach ($prop in $el.psobject.Properties) {
# iterate over properties
if ($dt = $prop.Value -as [datetime]) {
switch ($dt.Kind) {
'Utc' { $prop.Value = $dt.ToLocalTime() }
# Note: calling .ToLocalTime() is not an option, because it interprets
# an 'Unspecified' [datetime] as UTC.
'Unspecified' { $prop.Value = [datetime]::new($dt.Ticks, 'Local') }
}
}
elseif ($prop.Value -is [Array] -or $prop.Value -is [System.Management.Automation.PSCustomObject]) {
& $sb $prop.Value # recurse
}
}
}
}
# Start walking.
& $sb $args[0]
} $fromJson
# Output the transformed-in-place object graph
# that now contains only Local [datetime] instances.
$fromJson
