ClassMethod ConvertAthenaTimes(pDate As %String = "", pStartTime As %String = "", pDuration As %String = "", pTZ As %String = "") As %String
{
    //Set Defaults and Return if no Values Passed in
    Set return = ""
    q:pDate="" return
    q:pStartTime="" return
    //Convert pDuration into Seconds
    s:pDuration]"" pDuration=pDuration*60
    s dt=$zdth(pDate_" "_pStartTime_pTZ,1,5)
    if pDuration set $p(dt,",",2)=$p(dt,",",2)+pDuration
    set return=$zdt(dt_pTZ,3,5)

    // do something
    q return
}

ClassMethod TestFunction()
{
    try{
        w ##class(SERVICESVCPKG.Model.Functions.ProcessFunctions).ConvertAthenaTimes("7/21/2022","08:00",30,"-05:00")
    }catch(ex){
    #dim ex As %Exception.AbstractException
    w ex.DisplayString()
    }
}

I made a few tweaks and got to this:

2022-07-21T08:30:00+00:00

Not Quite sure how to get that TZ to change outside of changing the display format and just appending the pTZ parameter which would work. Not sure if $ZDATETIMEH handles time zone conversions when going from internal time or not. 

Thanks so much Ian! I'll give that a shot this morning!

Thank you both so much! Now to figure out how to do this with embedded objects. For my destination objects. 

I'd imagine I'd define in my parent class

Property ExternalAlias As SERVICESVCPKG.Model.IntSvc.ExternalAlias(%JSONFIELDNAME = "ExternalAlias", %JSONINCLUDE = "inout");

Class SERVICESVCPKG.Model.IntSvc.ExternalAlias Extends (%SerialObject, %JSON.Adaptor)
{

Parameter %JSONENABLED = 1;

Property TenantID As %String(%JSONFIELDNAME = "TenantID", %JSONINCLUDE = "inout");

Property SystemID As %String(%JSONFIELDNAME = "SystemID", %JSONINCLUDE = "inout");

Property ExternalType As %String(%JSONFIELDNAME = "ExternalType", %JSONINCLUDE = "inout");

Property ExternalID As %String(%JSONFIELDNAME = "ExternalID", %JSONINCLUDE = "inout");

}

Thank you all again, I swear I've been banging my head against this for like a week with 10 tabs of docs open.