udunits/src/main/java/ucar/units/OffsetUnit.java
public double fromDerivedUnit(final double amount)
            throws ConversionException {
        if (!(_unit instanceof DerivableUnit)) {
            throw new ConversionException(getDerivedUnit(), this);
        }
        return ((DerivableUnit) getUnit()).fromDerivedUnit(amount)
                - getOffset();
    }
public double fromDerivedUnit(final double amount)
            throws ConversionException {
        if (!(_unit instanceof DerivableUnit)) {
            throw new ConversionException(getDerivedUnit(), this);
        }
        return ((DerivableUnit) getUnit()).fromDerivedUnit(amount)
                - getOffset();
    }
Converts a value in the convertible derived unit to the equivalent value
in this unit.

@param amount
The value in the convertible derived unit.
@return The equivalent value in this unit.
@throws ConversionException
Can't convert between units.