NTPClient library giving incorrect date and time 1-1-1970.

hello,
i am doing practical to display current time and date in OLED 0.96 display using SPI communication with Nodemcu i have followed all the basic instruction but i am getting wrong date and time.initially i got correct date and time but after that i am getting output as 1-1-1970 can anyone suggest me what should i do.I am attaching my sketch below.

Thanks

demo.ino (5.49 KB)