Depending on the settings of the DAQ-Assistant (Min & Max Period), the timebase is calculated, and set to the optimal timebase the hardware provides. So there is no default timebase. You can read the selected timebase by wiring the DAQmx Channel Property Node (Found in the DAQmx Functions Palette) to the Task-Output of the DAQ-Assistant VI and Selecting the Property "Counter-Input-->General Properties-->Counter Timebase-->Rate". You can then read the value, and with the same property node you can also write a new value to this property and set a new timebase.
Hope this helps.
