I spent a bunch of time trying to find a way to add data to journal fields while maintaining the original time stamp and user ID for a customer migrating to ServiceNow and wanting to maintain their notes from their existing system. I suspect that this issue has been solved a number of times, however I didn't see it posted anywhere. Here is the code that I used to make this happen, this is based on my experience & no guarantees

insertJournalHistory: function(targetRecord, content, userName, createdDateTime) {

this.updateNumber = 1;

this.tableName = targetRecord.sys_class_name;

this.sysID = targetRecord.sys_id.toString()

this.createdDateTime = new GlideDateTime();

this.createdDateTime.setDisplayValue(createdDateTime,"MM-dd-yy HH:mm:ss");

this.journalField = new GlideRecord("sys_journal_field");

this.journalField.element = "comments";

this.journalField.element_id = this.sysID;

this.journalField.name = this.tableName;

this.journalField.sys_created_by = userName;

this.journalField.sys_created_on = this.createdDateTime;

this.journalField.value = content;

this.journalField.autoSysFields(false);

this.journalSysID = this.journalField.insert();

this.historySet = new GlideRecord("sys_history_set");

this.historyExists = this.historySet.get("id", this.sysID);

if (this.historyExists) {

this.historyID = this.historySet.sys_id.toString();

this.historySet.updates += 1;

this.historySet.update();

this.updateNumber = this.historySet.updates;

} else {

this.historySet.date_format = "yyyy-MM-dd HH:mm:ss";

this.historySet.id = this.sysID;

this.historySet.language = "en";

this.historySet.last_update_recorded = this.createdDateTime;

this.historySet.line_table = "sys_history_line0000";

this.historySet.sys_created_by = "system";

this.historySet.sys_created_on = this.createdDateTime;

this.historySet.sys_updated_by = "system";

this.historySet.sys_updated_on = this.createdDateTime;

this.historySet.table = this.tableName;

this.historySet.timezone = "America/Los_Angeles";

this.historySet.autoSysFields(false);

this.historyID = this.historySet.insert();

}

this.historyLine = new GlideRecord("sys_history_line");

this.historyLine.field = "comments";

this.historyLine.journal_sysid = this.journalSysID;

this.historyLine.label = "Additional comments";

this.historyLine.setValue("new", content);

this.historyLine.setValue("set", this.historyID);

this.historyLine.sys_created_on = this.createdDateTime;

this.historyLine.sys_updated_on = this.createdDateTime;

this.historyLine.update_time = this.createdDateTime;

this.historyLine.type = "audit";

this.historyLine.user_name = userName;

this.historyLine.update = this.updateNumber;

this.historyLine.autoSysFields(false);

this.historyLineID = this.historyLine.insert();

new GlideHistorySet(this.tableName, this.sysID);

},