If you don't need other features of Fit Y by X than box plots, you could try Graph Builder where box plots can be colored by a variable dragged to the Overlay Role.
Try the example code below. (The code demonstrates PMroz tip about the date format, but in my localization "m/y" has to be typed like "m-y".)
dt = New Table("example",
add rows(360),
New Column("date",
Numeric,
Format("m-y", 7),
formula(Date Increment(Informat("1-2009", "m-y"), "month", Mod(Row() - 1, 36)))
),
New Column("data", numeric, formula(Random Uniform())),
New Column("year", nominal, formula(Year(:date)))
);
dt << Graph Builder(Variables(X(:date), Y(:data), Overlay(:year)), Elements(Box Plot(X, Y, Legend(7))));