Dates and Eras:

The date column lists the unit's availability date.  For the vast majority of 
units, this is the date that it was first produced; for a tiny minority (which
might increase as time goes on) it is the date that unit first became used
by that faction.

Some dates have asterisks (*) after them.  These are approximations.  Over the
course of the many wars that have plagued humankind, records have been lost
and the exact date of manufacture of many designs has been lost from memory.

The era column is a more general version of the date column that lists the 
major era when each unit became available.  The eras are:

2570: Reunification War era
2750: Star League era
2950: Clans' "Golden Century"
3025: Inner Sphere Succession Wars
3050: Clan invasion of the Inner Sphere
Present: the current era

In general, units from earlier eras are available in later eras, with one 
exception: units in the 2750 era are only available to Comstar (and the
clans) during the 3025 and 3050 eras.