public Date getModificationTime() {
        if (!modificationTimeSet) {
            throw new IllegalStateException(PropertyUtil.getString("PNGEncodeParam21"));
        }
        return modificationTime;
    }
Returns the modification time to be stored with this image.

<p> If the bit depth has not previously been set, or has been
unset, an <code>IllegalStateException</code> will be thrown.

@throws IllegalStateException if the bit depth is not set.
plugins/svg-plugin/batik-codec-fix/src/main/java/org/apache/batik/ext/awt/image/codec/png/PNGEncodeParam.java