Show the current time of when a text is entered in a form

I wanted to request some assistance on how i could be able to have a chart I created show the current time when a text is entered. I have two columns one for the text entry and one for the time to show when the text was entered in but I'm using this below and it only show the current time when opening the document and not the time a text has been entered. Would appreciate some assistance. Thank you. 

this.getField("text1").value = (“text”); var f = this.getField("TimeStamp1");

if (f.valueAsString=="") f.value = util.printd("HH:MM", new Date());