Milliseconds since the Unix epoch for an orbit's epoch string, cached.
An ISO date-time with no time-zone designator is read as UTC rather than
as the viewer's local time (what new Date alone does), so a body's
position can't depend on where in the world the page is opened — see
OrbitalElements.epoch. NaN for an unparseable string (flagged by
validateSystemData as an invalid orbit).
Milliseconds since the Unix epoch for an orbit's
epochstring, cached. An ISO date-time with no time-zone designator is read as UTC rather than as the viewer's local time (whatnew Datealone does), so a body's position can't depend on where in the world the page is opened — see OrbitalElements.epoch. NaN for an unparseable string (flagged by validateSystemData as an invalid orbit).