Additions:

* Constructor accepts a boolean flag to enable the UTC mode: { utc: true }.
* Method utc() returns a clone of the instance in the UTC mode.
* Method local() returns a clone of the instance in the non-UTC (local) mode.
* Method isUTC() checks, if the instance is in the UITC mode.
* Method utcOffset() returns the time zone offset to UTC consistently with Date.prototype.getTimezoneOffset()

Differences to the "normal" (local) dayjs mode:

* Assume UTC as the time zone of the string passed to the constructor - for example, "2018-10-28" will be parsed as "2018-10-28T00:00:00Z". *But only if the time zone is omitted.* If you end the string tith a TZ offset - "2018-10-28T00:00:00+02:00", it will be retained and the whole date will be converted to UTC, when iniitalizing the datejs object.
* Methods returning the date parts like year(), month() etc. return UTC parts using Date.propertotype.getUTC* methods.
* Methods manipulating the date - set() - work with the UTC values of the date parts, as the getters above.
* The format() method always formats the time zone as "+00:00".
* The utcOffset() method always returns zero.

…stance in UTC mode