Date

[ZecaStevenson]

If you pass the exact sam METAR a couple of time, the resulting object will have a different "time" attribute each time:
https://skydivejkl.github.io/metar.js/#SBSP%20210000Z%2014006KT%209999%20-RA%20BKN007%20OVC013%2017/16%20Q1023

SBSP 210000Z 14006KT 9999 -RA BKN007 OVC013 17/16 Q1023
1st call: 2022-03-21T00:00:01.762Z
2ns call: 2022-03-21T00:00:34.040Z
3rd call: 2022-03-21T00:00:50.418Z

The problem seem to be here:
METAR.prototype.parseDate = function() {
this.next();
var d = new Date();
d.setUTCDate(asInt(this.current.slice(0, 2)));
d.setUTCHours(asInt(this.current.slice(2, 4)));
d.setUTCMinutes(asInt(this.current.slice(4, 6)));
this.result.time = d;
};

Also, I'm not sure if the METAR timezone is being taken into account. In this example SBSP is the São Paulo–Congonhas Airport which is in GMT-3.