ParseExact Exception [closed]

If the format of the input date-time string is the same as in your previous question on that topic, e.g. '12/16/2024 11:30:23 AM', then you can simplify your code to the following, taking advantage of the fact that this format is directly recognized by a [datetime] (System.DateTime) cast, irrespective of what culture is currently in effect (as explained in this answer to the linked question):

([datetime] $StartDate.ToString()).ToString('dd/MM/yyyy')

That said, $StartDate already appears to be a [datetime] instance in your code - given that you're calling .ToString() on it (and as since implicitly confirmed by an error message in one of your comments on the question) - so you can simply call (see the next section for details on the culture-related considerations):

# Assumes that $StartDate is a [datetime] instance.
$StartDate.ToString('dd/MM/yyyy')

# More robust alternative, assuming that the target culture is en-GB (UK English)
$StartDate.ToString('dd/MM/yyyy', [cultureinfo] 'en-GB')

# Even simpler (same assumption)
$StartDate.ToString('d', [cultureinfo] 'en-GB')

# Alternative that hard-codes the separators by \-escaping them.
# Note that this is only culture-invariant with *numeric* components.
$StartDate.ToString('dd\/MM\/yyyy')

If $StartDate is a string, after all:

• No need to call .ToString() on it.

• To deal with varying lengths of the date part due to single- vs. double-digit components (e.g. '1/7/2025' vs. '01/07/2025'), split the string at the first embedded space character instead of trying to extract a substring of fixed length:

  [DateTime]::ParseExact(
    (-split $StartDate)[0], 
    'M/d/yyyy', 
    $null
  ).ToString('dd/MM/yyyy') # See culture-sensitive considerations below.
  

Note:

• The unary form of PowerShell's -split operator splits the input string(s) into an array of substrings by any nonempty run of whitespace, and using [0] on the result returns the first substring.

• Custom date format M/d/yyyy is capable of also parsing double-digit month and day components (including 0-padded ones).

• You're relying on the current culture both for input parsing (due to using $null as the third argument) and for output formatting (due to passing only a format string, not also a culture, to .ToString(), even though you're clearly transforming from one culture-specific string representation to another.

  • The previously linked answer explains why specifying the culture context explicitly if it differs from the current one makes for a more robust solution; e.g., to transform to a UK English representation, replace .ToString('dd/MM/yyyy') above with
    .ToString('dd/MM/yyyy', [cultureinfo] 'en-GB')
    or, more simply - given that in said culture that format is the default short date pattern, expressed as standard date-time format string d,
    .ToString('d', [cultureinfo] 'en-GB')
    Alternatively - as long as the date-component placeholders solely refer to numeric (rather than symbolic) date components - you can hard-code the separators to ensure their verbatim use, by \-escaping them:
    .ToString('dd\/MM\/yyyy')