How to reformat the date on files in bulk using powershell - powershell

Question

I need to format the file name from ...
2639423_3_30_56 PM_9_4_2020.txt
... to ...
2639423-15-30-56-09-04-2020.txt
i.e. Need to change date in Military time format and replace '_' with '-', Also append with “0” for single digit months and single digit days
Please advise I need to perform this in powershell & need to perform this in bulk.

Answer 1

Start by splitting the file name into two parts - the prefix, which remains the same, and the timestamp, which you want to re-format:
$basename = '2639423_3_30_56 PM_9_4_2020'
$prefix,$timestamp = $basename -split '_',2
Next, parse the timestamp according to it's specific format:
$inputFormat = 'h_mm_ss tt_d_M_yyyy'
$parsedDateTime = [datetime]::ParseExact($timestamp,$inputFormat,$null)
Finally convert the parsed [datetime] object back to a string with the desired output format, and then join the prefix and (updated) timestamp together again:
$outputFormat = 'HH-mm-ss-dd-MM-yyyy'
$timestamp = $parsedDateTime.ToString($outputFormat)
# or
$timestamp = Get-Date $parsedDateTime -Format $outputFormat
$newFileName = $prefix,$timestamp -join '-'
# 2639423-15-30-56-09-04-2020
To rename the files in bulk, pipe the files to Rename-Item and use the parameter binder to generate the new name of each file based on the existing name:
Get-ChildItem -Path .\folder\with\files -Filter *.txt |Rename-Item -NewName {
$prefix,$timestamp = $_.BaseName -split '_',2
$parsedDateTime = [datetime]::ParseExact($timestamp, 'h_mm_ss tt_d_M_yyyy', $null)
$timestamp = $parsedDateTime.ToString('HH-mm-ss-dd-MM-yyyy')
$newBaseName = $prefix,$timestamp -join '-'
$newBaseName + $_.Extension
}