When converting a specific datetime value to GMT or UTC the output is a bit confusing:
moment.tz("1751-01-01T00:00:00", "Europe/Dublin").clone().tz("Europe/London").format("YYYY-MM-DDTHH:mm:ss.SSSS") -> "1751-01-01T00:25:21.0000"
or this
moment.tz("1751-01-01T00:00:00", "Europe/Dublin").clone().utc().format("YYYY-MM-DDTHH:mm:ss.SSSS") -> "1751-01-01T00:25:21.0000"

The first thing I learned here is that the 25min added is actually not a bug :)
But what about the extra 21 seconds added in both cases?? Is that a bug?