import sys
from PyQt5 import QtCore, QtGui, QtWidgets

class MyForm(QtWidgets.QDialog):
    def __init__(self):
        super().__init__()

        self.dateEdit = QtWidgets.QDateEdit(self)
        self.dateEdit.setDateTime(QtCore.QDateTime(QtCore.QDate(2019, 2, 23),
                                  QtCore.QTime(0, 0, 0)))
        self.dateEdit.setDate(QtCore.QDate(2019, 2, 23))
        self.dateEdit.setDisplayFormat("dd/MM/yyyy")

        self.label = QtWidgets.QLabel()
        self.label.setText("{}".format(self.dateEdit.dateTime().toString('dd-MM-yyyy')))

        self.button = QtWidgets.QPushButton('Run')
        self.button.clicked.connect(self.onClick)

        layout = QtWidgets.QVBoxLayout(self)
        layout.addWidget(self.label)
        layout.addWidget(self.dateEdit)
        layout.addWidget(self.button)

    def onClick(self):
        firstDayText = '{}-01-01'.format(self.dateEdit.dateTime().toString('yyyy'))
        firstDay = QtCore.QDateTime.fromString(firstDayText, "yyyy-MM-dd")
        numDay = firstDay.daysTo(self.dateEdit.dateTime())
        self.label.setText("Выбрана дата {}, с начала года прошло {} дня/ей"
            "".format(self.dateEdit.dateTime().toString('dd-MM-yyyy'), numDay))


if __name__=="__main__":
    app = QtWidgets.QApplication(sys.argv)
    myapp = MyForm()
    myapp.show()
    sys.exit(app.exec_())