PapaDos wrote:
=ROUND( a1, 0 ) - AND( MOD( a1, 1 ) = 0.5, ISEVEN( a1 ) )

That was my answer. But to avoid the anomalous failure of MOD() with
large numbers, try:

=round(a1,0) - and(a1-int(a1)=0.5, iseven(a1))