LWC Javascript Datetime default value

時刻項目のディフォルト値設定する。

HTML:
<lightning-input-field required field-name="StartTime__c" value={starttime}></lightning-input-field>

        

JS:
//20211013093000
this.starttime = new Date(message.dateselected.substr(0,4), 	//YYYY
	    message.dateselected.substr(4,2)-1, 		//MM  month index 0-based
	    message.dateselected.substr(6,2), 			//DD
	    message.dateselected.substr(8,2), 			//HH
	    message.dateselected.substr(10,2), 			//MM
	    message.dateselected.substr(12,2)).toISOString(); //SS