555 timer with output voltage equal to 5V?
The output voltage will never quite reach the supply voltage. From your datasheet: 



With a 10mA load to +5 you'll see 100mV or more drop when sinking 10mA, and about 1.45V drop (meaning output voltage 3.55V typically) when sourcing 10mA (load to ground). 

If you need more voltage than that when sourcing a low current (such as 1mA) you can use a CMOS 555 which only drops a couple hundred mV. 

At high current you could add output transistors such as power MOSFETs that might drop a few mV for a big one at moderate current, but never zero.