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# Microsoft Windows [Version 10.0.17134.648] 
# powershell 5.1.17134.48
# parse Notepad++ "backup on save" filenames.
# dir directory. Include index*,avail* where the last write time is less than
# 4 days. Display a parsed Name. Display the Length (size) and the LastWriteTime.
# Can be used as a shortcut: powershell -noexit $time = (Get-Date).AddDays(-4); gci * -include index*,avail* | where $_.LastWriteTime -gt $time | Format-Table @n='foo';e=$_.Name -replace '(?<=^.*.html).*$',Length,LastWriteTime -HideTableHeaders
# Start it in your directory of choice.
$time = (Get-Date).AddDays(-4)
gci * -include index*,avail* | where $_.LastWriteTime -gt $time | Format-Table @n='foo';e=$_.Name -replace '(?<=^.*.html).*$',Length,LastWriteTime -HideTableHeaders

outputs:

available13.html 93130 3/26/2019 8 : 10 : 05 AM
index.html 39386 3/26/2019 8 : 10 : 05 AM 

If I use:

$time = (Get-Date).AddDays(-4)
gci * -include index*,avail* | where $_.LastWriteTime -gt $time | format-table -HideTableHeaders Name,Length,LastWriteTime

I get:

available13.html.2019-03-26_081523.bak 93130 3/26/2019 8 : 10 : 05 AM
index.html.2019-03-26_081538.bak 93134 3/26/2019 8 : 15 : 23 AM

Microsoft Windows [Version 10.0.17134.648]

powershell 5.1.17134.48

Parse NPP / Notepad++ / Notepad Plus "backup on save" filenames.

dir directory. Include index*,avail* where the last write time is less than 4 days. Display a parsed Name. Display the Length (size) and the LastWriteTime.

Can be used as a shortcut:
powershell -noexit $time = (Get-Date).AddDays(-4); gci * -include index*,avail* | where $_.LastWriteTime -gt $time | Format-Table @n='foo';e=$_.Name -replace '(?<=^.*.html).*$',Length,LastWriteTime -HideTableHeaders
Start it in your directory of choice.

I use Notepad++ "backup on save" feature to bill my clients for time I work on their website. This script creates a formatted table output with the filenames, size, date and the extent of the time the file was worked on.

See LotPings answer below for information about the name parse.

I'd use a RegEx with zero length lookbehind assertion to remove everything after html from $_.Name

This can be done with a calculated property either in a Select-Object or also in a Format-*

Get-ChildItem -File | 
 Format-Table @n='foo';e=$_.Name -replace '(?<=^.*.html).*$',Name -HideTableHeaders

Sample output:

available13.html available13.html.2019-03-26_081523.bak
index.html index.html.2019-03-26_081538.bak

You can add a new property with Add-Member like this

$time = (Get-Date).AddDays(-4)
$files = gci * -include index*,avail* | where $_.LastWriteTime -gt $time
foreach ($f in $files) Add-Member noteproperty newName -Value `
 $f.Name.Substring(0, $f.Name.Length - ".yyyy-mm-dd_iiiiii.bak".Length)

$files | Format-Table -HideTableHeaders newName,Length,LastWriteTime

Note that the above snippet assumes that your names always end with .yyyy-mm-dd_iiiiii.bak. If they have some other format then you must include that information in the question, and you may need to use other string methods like replace, substring... to remove the unnecessary part