Are you describing a celestial event, or scheduling a New Year’s party for humans? For the former, you are probably already using TAI (or GPS). For the latter,

Are you describing a celestial event, or scheduling a New Year’s party for humans? For the former, you are probably already using TAI (or GPS). For the latter, use an ISO 8601 string, and call it Europe/London instead of GMT. As you recognize - the mapping of human dates to celestial time is unstable. Don’t pretend it is by using a number until you need to.