Getting Timestamps in arduino uno and sending that to master from slave in SPI

Hallo,

Has any body know how do I get PC time or real time in arduino uno microcontroller without any external devices. I need to send time stamps and sensor values from slave to my master in SPI communication.

without any external devices.

Wouldn't the PC be an external device?

I need to send time stamps and sensor values from slave to my master in SPI communication.

Time stamps relative to what?