how to set xdate format from " May 6 2008" to "2008-05-06"?

[songroom2016]

trafficstars

using PyPlot;
clf();
close();
fig =figure();
ax = fig[:add_subplot](111)  
x = [Date(now()) + Dates.Day(i) for i in collect(1:100)] 
y = rand(100) 
ax[:plot](x,y, color="red", linewidth=2.0, linestyle="-") 

but i get x autofmt_xdate ,format such as " May 6 2008" how to set xdate format as "2008-05-06"? thanks.