Get-Date -UFormat inconsistencies compared to Linux strftime

With reference to #4264 first reported by @chuanjiao10 - on further examination there are a number of UFormats outputs which diverge from the output under Linux. Because the UFormat parameter is supposed to be UNIX format, the output should match that of the date command (i.e. strftime) on Linux.

It's clear that some of the divergences are simple format issues, for instance leading zeros. Others are down to following the ISO 8601 week date specification (https://en.wikipedia.org/wiki/ISO_week_date). And others appear to be differences between locale data on Linux and .Net. In these latter cases it looks like the C locale is followed, which may actually be the best that can be done given the lack of Linux locale data.

# %c

- [x] (PR #4805) The preferred date and time representation for the current locale.

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %c                                                   
```

```none                                                       
Sat Jan  1 01:00:00 2005
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%c
```
```none                                                       
Sat 01 Jan 2005 01:00:00 AM GMT
```

### C

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %c                                                   
```

```none                                                       
Sat Jan  1 01:00:00 2005
```

#### Linux
```bash
LC_TIME="C.UTF-8" date -d 2005-01-01T01:00:00 +%c
```

```none
Sat Jan  1 01:00:00 2005
```

---
# %g

- [x] (PR #14555) Like %G, but without century, that is, with a 2-digit year (00-99).

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %g                                                     
```

```none                                                       
05
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%g
```

```none
04
```

---
# %G

- [x] (PR #14555) The ISO 8601 week-based year ... with century as a decimal number.

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %g                                                     
```

```none                                                       
2005
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%G
```

```none
2004
```

---
# %j

- [x] (PR #4805) The day of the year as a decimal number (range 001 to 366).

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %j                                                    
```

```none                                                       
1
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%j
```

```none
001
```

---
## %k

- [x] (PR #4805) The hour (24-hour clock) as a decimal number (range 0 to 23); single digits are preceded by a blank.

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %k                                                    
```

```none                                                       
01
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%k
```

```none
 1
```

---
# %l

- [x] (PR #4805) The hour (12-hour clock) as a decimal number (range 1 to 12); single digits are preceded by a blank.

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %l                                                   
```

```none                                                       
01
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%l
```

```none
 1
```

---
# %s

- [x] (PR #4805) The number of seconds since the Epoch, 1970-01-01 00:00:00+0000 (UTC).

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0 -Millisecond 0  -UFormat %s                                                 
```

```none                                                       
1104541200
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%s
```

```none
1104541200
```

```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0 -Millisecond 1 -UFormat %s                                                 
```

```none                                                       
1104541200.001
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00.001 +%s
```

```none
 1104541200
```

---
# %u

- [x] (PR #14549) The ISO 8601 day of the week 1-7 (M-Su).

### en-US

#### PowerShell

```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 2 -Hour 1 -Minute 0 -Second 0  -UFormat %u
```

```
0
```

#### Linux

```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-02T01:00:00 +%u
```

```
7
```

---
# %U

- [ ] The week number of the current year as a decimal number, range 00 to 53, starting with the first Sunday as the first day of week 01.

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %U                                                   
```

```none                                                       
0
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%U
```

```none
00
```

---
# %V

- [ ] The ISO 8601 week number ... of the current year as a decimal number, range 01 to 53, where week 1 is the first week that has at least 4 days in the new year.

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %V                                                   
```

```none                                                       
1
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%V
```

```none
53
```

---
# %W

- [ ] The week number of the current year as a decimal number, range 00 to 53, starting with the first Monday as the first day of week 01.

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %W                                                   
```

```none                                                       
0
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%W
```
```none                                                       
00
```

---
# %x

- [ ] The preferred date representation for the current locale without the time.

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %x                                                 
```

```none                                                       
01/01/05
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%x
```
```none                                                       
01/01/2005
```

### C

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %x                                                 
```

```none                                                       
01/01/05
```

#### Linux
```bash
LC_TIME="C.UTF-8" date -d 2005-01-01T01:00:00 +%x
```
```none                                                       
01/01/05
```

---
# %X

- [ ] The preferred date representation for the current locale without the time.

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %X                                                 
```

```none                                                       
01:00:00
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%X
```
```none                                                       
01:00:00 AM
```

### C

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %X                                                 
```

```none                                                       
01:00:00
```

#### Linux
```bash
LC_TIME="C.UTF-8" date -d 2005-01-01T01:00:00 +%X
```
```none                                                       
01:00:00
```

---
# %Z

- [ ] The timezone name or abbreviation.

### en-US

#### PowerShell
```powershell
[System.Threading.Thread]::CurrentThread.CurrentCulture = [System.Globalization.CultureInfo]::GetCultureInfo('en-US') 
get-date -Year 2005 -Month 1 -Day 1 -Hour 1 -Minute 0 -Second 0  -UFormat %Z                                                
```

```none                                                       
+00
```

#### Linux
```bash
LC_TIME="en_US.UTF-8" date -d 2005-01-01T01:00:00 +%Z
```
```none                                                       
GMT
```



```powershell
> $PSVersionTable
Name                           Value                                           
----                           -----                                           
PSVersion                      6.0.0-beta                                      
PSEdition                      Core                                            
GitCommitId                    v6.0.0-beta.5                                   
OS                             Linux 4.10.0-32-generic #36~16.04.1-Ubuntu SM...
Platform                       Unix                                            
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}                         
PSRemotingProtocolVersion      2.3                                             
SerializationVersion           1.1.0.1                                         
WSManStackVersion              3.0  
```

I initially, naively, made PR #4508 for #4264 which simply used the **GetWeekOfYear** method from the Gregorian Calendar. Unfortunately that is too simplistic, and the [formula](https://en.wikipedia.org/wiki/ISO_week_date) needs to be followed to determine the correct week number. I suggest that PR be closed and new PRs to be created against this tracking issue. My suggestion is that the Pester tests be broken out into a test per UFormat to make this easier (with the failing tests commented until they can be addressed).

Get-Date -UFormat inconsistencies compared to Linux strftime #4750

Description

%c

en-US

PowerShell

Linux

C

PowerShell

Linux

%g

en-US

PowerShell

Linux

%G

en-US

PowerShell

Linux

%j

en-US

PowerShell

Linux

%k

en-US

PowerShell

Linux

%l

en-US

PowerShell

Linux

%s

en-US

PowerShell

Linux

Linux

%u

en-US

PowerShell

Linux

%U

en-US

PowerShell

Linux

%V

en-US

PowerShell

Linux

%W

en-US

PowerShell

Linux

%x

en-US

PowerShell

Linux

C

PowerShell

Linux

%X

en-US

PowerShell

Linux

C

PowerShell

Linux

%Z

en-US

PowerShell

Linux

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