If you just want to set the initial date the picker comes up with, then you just want the value
config attribute of the picker.
{
xtype: 'datepickerfield',
value: (new Date()), // the value in the actual input (will also be in the picker UI)
picker: {
value: (new Date()), // use this if you DON'T want/have a value in the actual input
yearFrom: (new Date()).getFullYear(),
yearTo: 1930
}
}
Read more in the documentation (especially the "value" and "picker" config options). And here'a a Sencha Fiddle demonstrating usage.