To be able to set a date field you need to set it using a correct date format
document1.setValue("DateField","01.01.1970")
The Date format needs to be java.util.Date so try this
document1.setValue("DateField",new Date("01.01.1970"))
But I would suggest doing it this way because then you code isn't bound to that the server is using that specific locale settings.
var date=new Date();
date.setFullYear(1970)
date.setMonth(0) //remember months starts with 0
date.setDate(1)
document1.setValue("DateField",date)
One thing is that I haven't got this to work when running in a Notes client, but that might be a bug.