do case
case mod(nYear, 400) = 0
  lLeapYear = .t.
case mod(nYear, 100) = 0
  lLeapYear = .f.
case mod(nYear, 4) = 0
  lLeapYear = .t.
otherwise
  lLeapYear = .f.
endcase

Or This...

Leapyear = MOD(nYear,4)=0 AND (MOD(nYear,100) # 0 OR MOD(nYear,400)=0)

I would also vote for the former, although they do both give the same result.