Confidence: It's likely to be this, but I can't be certain.
Notes: Shape and pattern closely match book picture. Dark grey cross band with some greenish content and areas above and below greenish. The banding close to the outer edge of the forewing also closely matches the book picture and there appear to be two faint and narrow cross bands towards the base of the forewings. The uncertainty with this identification is that the flight time is given as May-June so this specimen is quite early.