First, I think, you should check the above method...
You use the date variable two times

set date = $piece(...)
return date_" "...

I'm pretty shure, if the time in Vienna (Europe) is 00:30:00 then the coresponding UTC time is 23:30:00 previous day (standard time). But you never change the date variable!
As a timestamp:
2020-08-31T00:30:00+01:00 (Vienna, 31. August)
2020-08-30Z23:30:00 (UTC,  30.August)

Second (but this is just my very privete view), checking time(stamp) format should be done BEFORE calling the ConvertW3CToTimestampUTC() method and not in the method itself.

The holy trinity of programming is:
1) accept data (data input)
2) check the data
3) if the data is OK use it, else back to input (or return an error)

If you accpet data without checking, it means, every time, you intend to use those data, you have to check them again and again (as above), wich is not very efficient.

What about?

select TO_CHAR(TO_DATE('5/8/2020','MM/DD/YYYY'),'YYYY-MM-DD')

Of course works, because the format part MATCHES the date part!

Did you read the documentation?

TO_DATE(datestring [,format])

Your datestring is 5/8/2020 but your format is 'YYYY-MM-DD' which does not match '5/8/2020'!!

As always, it depends on ...
In this case on the definition of the date column.

a) if the date column is defined as %Date (i.e.: Property DateOfBirth As %Date) then the STORED value is just a number like 65567, because dates are stored in $Horolog format). 

b) if the date column is defined (for whatever reason) as %String (i.e.: Property DateOfBirth As %String) then the stored value is (as the type says) a string, and you can use the pattern operator to check the format.So the first question is, how is your date column defined?

You are a lucky man, you own a 300bd device!
I have one, but it's a newer type with 2400bd BUT with a fallback mode for 300bd too (if I recall it right).

Thanks for the (finger board) link.

Until now, I didn't realise, that the tags are clickable. Sorry.
But in general, it's a good idea, before using an acronym, to spell it out first, to avoid misunderstandings

radix 10: mantisse *  10**exponent  (the  ISC  floating format)

radix 2 : mantisse * 2 ** exponent ($double/ IEEE 754 format )

Just to bring things in the right light

I'm a retired person but one needs some level of activity, that's the reason, why I'm here, do give answers and comments (and if the old customers need "a kind of prolonged help", I'm there too).
In early and mid seventies I programmed some so called "desktop computer", learned Cobol, IBM/360 assembler, Algol60 and Pascal as the preferred language on the university. Then, end of 1977 I met M (DEC's V4B).
Until the nineties thre were no IDEs, the first one I used was Serenji (from GeorgeJames) for MSM and Cache's Studio (with a lovely meat-chopper as the compiler icon). And there were no GitHub.
Anyway, we had great projects and great programms and fun.

The above means, I for myself do not need anything, neither Atelier nor VS-Code not even Studio but in case, I want to edit a class or routine, I prefer Studio.
During my active times, Studio was a great editor (and still it is!), sometimes I had more then one Studio open (at the same time, of course) to different Servers or different namespaces that's the reason why I requested the feature (server defined colors) some decades ago. The light red was the connection into, say a productio system, the green was into  a test namespace and blue, for example was the developmet system. A short glimpse at the display, if I saw red, my brain said, "keep fingers from keyboard", if I saw blue then my brain was ready for switch off... 

But I know several people working since decades with Studio only (and with the line editor before - I hope you know ^% and remember it?). Those people are (roughly) between 50 and 60 years old, worked (and still work) with whatever brand of M-Language (DTM, MSM, ISM, GT-M, Cache etc.) and never had contact to "modern" languages like java, C++, javascript, etc.
That's in my opinion the reason, why Atelier not so widespread as it deserved to be.

De facto, we have a two class society of M-programers

a) the young ones,
- they know everything about the current IDEs (Visual Studio, VS-Code, Eclipse, etc.
- they try to master (more or less) the M-World but
- their heart beats for languages as Java, Javascript, C++, Angular, Scala, etc.
- can't live without Git, GitHub, Bitbucket etc.
- they do not understand the essence/character of M-language (especially Cache/Iris)
- and think, MongoDB is the only NoSql-DB
- work few months (or one or two years) for a company, then change
- in the new company start over with another environment (Java, Angular, whatever)
- many are, I use to say, single-threaded: one project at time only
- etc.

b) and the old ones
- they grew up with the M-language
- they understand the power of M as a unity (language and database)
- most of them never used Git (and similar sites) for version control
- some of them never used a version control at all!
- most of them work with Windows (but this has historical reason)
- a few years before retirement there's no incentive to change to new IDEs

The real problem is not which IDE is in use, the real problem is to get people for M-programming, who then use an IDE. There are, I think, more old(er) then young M-Programer. I know for companies, they search for M-Programers but have dufficulties to find one.
That said, at the end of the day, you have to raise interesse for M-Language and M-Database. So you will need another means and not just a new IDE.
Maybe I'm wrong, but it's like comparing the M-Language and M-Database with a dinner by a start chef.
Nobody asks for the IDE they was in use, nobody asks for the pots and stoves they was in use.
After you leave the restaurant, the only thing that remains is, was the dinner good, were you satisfied.
Same goes the M-Language, does it can all things I need, will I be happy with it.
And yes, my answer is a big yes, definitive.

In my opinion the very first question is the support. a) Studio

• it's installed (and updated) with Cache/IRIS
• if I have a question, there is WRC b) VS-Code
• it's not installed with Cache/IRIS
• so I have to care about the installation
• where to get the suitable files (and updates)
• and I have to care about the "how to" install it
• in a case of a problem, who am I contacting?
  1. the WRC? They will say, it's not our product
  2. the community? Are they always (24/7) there?

The current situation with VS-Code and the COS-plugin (my very private opinion) is

1. it is assumed, that one is an experienced user of VS-Code
2. there are no installinstuctions and user manuals

ad 1): assume, one day ISC says "VS-Code with COS plugin exists, so we can drop Studio". What should happen to all those COS-programers, how should they master VS-Code? Most of them have never ever heard about VS-Code? Keep in mind, not everybody grew up with mother's breast in one hand and an App in another !

ad 2): yes, there are some YouTube videos. On all videos I saw, VC-Code was already installed

Do not get me wrong, I'm not again VS-Code (the truth is, I would like to use it on my Linux) and I'm not for keeping Studio until eternity (because there is no Studio for Linux). But currently, Studio has some advantage (again, for me):

• it's installed
• I can have (and yes, I have) several connections
• I can give each connection a different (background) color
• I have the WRC (but I never had a Studio issue)

The %OpenId() method checks, if the object to be opened ALREADY open (in the current session/job). If it's open then it just returns with an success.  In other case, it loads it from the disk. 

If on your development system, for whatever reason,  there is an open instance  then you save 6.1 million times the "load" operation.

You could try this way:

Class My.Table2 Extends %Persistent
{

Property Name As %String;

Property Age As %Numeric;

Property City As %String;

Property Phone As %String;

ClassMethod Test()
{
    // This is your Data-Object...
    set data=[]
    do data.%Push({"Name":"Joe", "Age":44, "City":"Boston", "Phone":"1-234-4567"})
    do data.%Push({"Name":"Ron", "Age":48, "City":"Dallas", "Phone":"1-234-5678"})
    do data.%Push({"Name":"Eve", "Age":40, "City":"Miami",  "Phone":"1-234-4567"})
    
    do data.%Push($lb("Tommy", 50, "New York", "1-345-6789"))
    do data.%Push($lb("Alexa", 35, "Portland", "1-567-8901"))

    // Now insert all the above data into your table...
    if 'data.%Size() quit
    
    set cnt=0, size=data.%Size()
    while $i(cnt)<=size {
        set rowData=data.%Get(cnt-1)
        &sql(
            INSERT INTO My.Table2 (Name,Age,City,Phone)
            VALUES (My.Table2_DataProvider(:rowData), :row(2), :row(3), :row(4))
        )
    }
} 

ClassMethod DataProvider(rowData) As %String [ PublicList = row, SqlProc ]
{
    kill row
    if $isobject(rowData) {
        set it=rowData.%GetIterator()
        while it.%GetNext(,.val) { set row($i(row))=val } 

    } else {
        for i=1:1:$ll(rowData) { set row(i)=$lg(rowData,i) }
    }
    quit row(1)
}

Storage Default
{
<Data name="Table2DefaultData">
<Value name="1">
<Value>%%CLASSNAME</Value>
</Value>
<Value name="2">
<Value>Name</Value>
</Value>
<Value name="3">
<Value>Age</Value>
</Value>
<Value name="4">
<Value>City</Value>
</Value>
<Value name="5">
<Value>Phone</Value>
</Value>
</Data>
<DataLocation>^My.Table2D</DataLocation>
<DefaultData>Table2DefaultData</DefaultData>
<IdLocation>^My.Table2D</IdLocation>
<IndexLocation>^My.Table2I</IndexLocation>
<StreamLocation>^My.Table2S</StreamLocation>
<Type>%Library.CacheStorage</Type>
} }

Create your data for insert and then

INSERT into yourtable (Prop1, Prop2, ...)

VALUES (sqlProcForTheFirstValue(), :localVarForOtherValues(2), :localVatForOtherValues(3),...)

see the above example.

Take care of the sequence of INSERT names and row(i) values.