How to program a one second timer using external crystal frequency ?

I dunno how to program this timer. I have a ATMEL T89C51RD2 Microcontroller have 2 mode for internal timing
(1)40Mhz for standard mode(default)
(2)and 20Mhz in X2 Mode.
However, I am using a 16Mhz as an exertnal frequency.

Problem
(1)I need to program a one second timer.

How to do it????
Help!!

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