significance: base multiple (defaults to 1 for x > 0 and -1 for x <0)
FLOOR(x,significance) is the nearest multiple of significance whose absolute value is at most ABS(x)
This function is Excel compatible.
FLOOR(x) is exported to ODF as FLOOR(x,SIGN(x),1). FLOOR(x,significance) is the OpenFormula function FLOOR(x,significance,1).