The DAC output of the Due is limited to 0.7V-2.5V. To turn that into any kind of useful signal you will need analog circuits such as opamps. What voltage do you want? What frequency?
What do you mean "variable amplitude DC"? If it's DC then it doesn't change and it's not variable. You might specify that you need an output variable between 0-5V referenced to ground and the maximum frequency is 5Hz (this means it can go from 0 to 5 OR 5 to 0 in one tenth of a second.) How much power do you need for this signal? 3mA? 30mA?
The SAM3X has a real-time-clock and Ethernet but it's completely useless on the Due board because those pins are not accessible without some extremely fine soldering skills. The DAC is not useless but it takes some work to make it useful.