Both of those suggestions seem to have the same effect as what I already had. Maybe I'm using them incorrectly.
This is my old code:
var birthdate = new Date("January 31, 2004 08:30:00");
var now = new Date();
var milsec = (now.getTime() - birthdate.getTime());
var secgap = milsec / 1000;
secgap = Math.round(secgap*10)/10;
document.ageform.secage.value = "Age in seconds: " + secgap;
var timeout = setTimeout("secage()", 50);
In which I replaced secgap = Math.round(secgap*10)/10; with your suggestions.