Hi @Smythe Smythee 

The best way i guess is to have the FHIR package installed and use the OOB transformations.
an alternative way would be something as follows;
 

ClassMethod TestHL7ToJson()
{
        set hl7FilePath = "C:\Intersystems\file.hl7"#dim hl7Msg As EnsLib.HL7.Message
        set hl7Msg=##class(EnsLib.HL7.Message).ImportFromFile(hl7FilePath, .sc)
        set tMSH=hl7Msg.getSegmentByIndex(1)
        set tName=tMSH.GetValueAt(9,":_~\&")
        
        set hl7Msg.DocType = "2.5:"_tName
        
        //Anticipating the PID element in line 3set tPID = hl7Msg.getSegmentByIndex(3)
        
        set jsonObj= {}
        do jsonObj.%Set("PatientID",tPID.GetValueAt(2))
        do jsonObj.%Set("PatientIdentifierList",tPID.GetValueAt(3))
        do jsonObj.%Set("AlternatePatientIDPID",tPID.GetValueAt(4))
        do jsonObj.%Set("PatientName",tPID.GetValueAt(5))
        do jsonObj.%Set("MothersMaidenName",tPID.GetValueAt(6))
        do jsonObj.%Set("DateTimeofBirth",tPID.GetValueAt(7))
        do jsonObj.%Set("AdministrativeSex",tPID.GetValueAt(8))
        do jsonObj.%Set("PatientAlias",tPID.GetValueAt(9))
        do jsonObj.%Set("Race",tPID.GetValueAt(10))
        do jsonObj.%Set("PatientAddress",tPID.GetValueAt(11))
        do jsonObj.%Set("CountyCode",tPID.GetValueAt(12))
        do jsonObj.%Set("PhoneNumberBusiness",tPID.GetValueAt(14))
        do jsonObj.%Set("PrimaryLanguage",tPID.GetValueAt(15))
        do jsonObj.%Set("MaritalStatus",tPID.GetValueAt(16))
        do jsonObj.%Set("Religion",tPID.GetValueAt(17))
        do jsonObj.%Set("PatientAccountNumber",tPID.GetValueAt(18))
        do jsonObj.%Set("SSNNumberPatient",tPID.GetValueAt(19))
        do jsonObj.%Set("DriversLicenseNumberPatient",tPID.GetValueAt(20))
        do jsonObj.%Set("MothersIdentifier",tPID.GetValueAt(21))
        do jsonObj.%Set("EthnicGroup",tPID.GetValueAt(22))
        do jsonObj.%Set("BirthPlace",tPID.GetValueAt(23))
        do jsonObj.%Set("MultipleBirthIndicator",tPID.GetValueAt(24))
        do jsonObj.%Set("BirthOrder",tPID.GetValueAt(25))
        do jsonObj.%Set("Citizenship",tPID.GetValueAt(26))
        
        W jsonObj.%ToJSON()
}

the above code generates a json for PID as follows;

{
    "PatientID": "1111111^^^TEST^MR",
    "PatientIdentifierList": "6910315054084^^M10^RSAID^GOV",
    "AlternatePatientIDPID": "",
    "PatientName": "Geoe^Schz^^MR^^^^^^^^^^.",
    "MothersMaidenName": "",
    "DateTimeofBirth": "19330722",
    "AdministrativeSex": "M",
    "PatientAlias": "",
    "Race": "",
    "PatientAddress": "GRASRK^CAPE TOWN^AA^^0001",
    "CountyCode": "",
    "PhoneNumberBusiness": "0",
    "PrimaryLanguage": "E^English",
    "MaritalStatus": "",
    "Religion": "",
    "PatientAccountNumber": "",
    "SSNNumberPatient": "",
    "DriversLicenseNumberPatient": "",
    "MothersIdentifier": "",
    "EthnicGroup": "",
    "BirthPlace": "",
    "MultipleBirthIndicator": "N",
    "BirthOrder": "",
    "Citizenship": ""
}

@Yuri Marx 
 

W$TR($ZDATETIME($HOROLOG,3,1)," ","T")

//adapted from (HS.FHIRServer.Utils).ConvertToUTC#dim tSecondsPerDay = 86400// time with offsetset tTimeWithOffset = $ZDATETIME($HOROLOG,3,5)

if (tTimeWithOffset [ "-") { set tOffsetSign = "-" }
    elseif (tTimeWithOffset [ "+") { set tOffsetSign = "+" }
    else { set tOffsetSign = "" }
    
    
if (tOffsetSign'="") {
    // compute offset in secondsset tOffset = $P(tTimeWithOffset,tOffsetSign,2)
    set tOffsetHrs = $P(tOffset,":")
    set tOffsetMins = $P(tOffset,":",2)
    set tOffsetInSeconds = (3600*tOffsetHrs) + (60*tOffsetMins)
    // get date/time in $H formatset tTime = $P(tTimeWithOffset,tOffsetSign)
    set tDateTimeH = $ZDTH(tDate_"T"_tTime,3,1)
    set tDateH = $P(tDateTimeH,",")
    set tTimeH = $P(tDateTimeH,",",2)
    // UTC time = timestamp value (+/-) offset// might have to increment or decrement the dateset tUTCDateH = tDateH
    if (tOffsetSign = "-") {
        set tUTCTimeH = tTimeH + tOffsetInSeconds
        if (tUTCTimeH >= tSecondsPerDay) {
            set tUTCDateH = tUTCDateH+1set tUTCTimeH = tUTCTimeH-tSecondsPerDay
        }
    }
    else { 
        set tUTCTimeH = tTimeH - tOffsetInSeconds
        if (tUTCTimeH < 0) {
            set tUTCDateH = tUTCDateH-1set tUTCTimeH = tUTCTimeH+tSecondsPerDay
        }
    }

    Set pDateTimeH = tUTCDateH_","_tUTCTimeH
    quit$TR($ZDT(pDateTimeH,3,1)," ","T")_"Z"
}