Every clock in Ardour has four different, selectable clock modes. Each mode displays time using different units. You can change the clock mode by Right-clicking on the clock and selecting the desired mode from the menu. Some clocks are entirely independent of any other clock's mode; others are linked so that changing one changes all clocks in that group. The different modes are:
New values for the clock can be typed in after clicking on the relevant clock. Clicking on the clock will show a thin vertical cursor bar just to the right of the next character to be overwritten. Enter time in the same order as the current clock mode—if the clock is in Timecode mode, you need to enter hours, minutes, seconds, frames. So, to change to a time of 12:15:20:15 you would type 1 2 1 5 2 0 1 5. Each number you type will appear in a different color, from right to left, overwriting the existing value. Mid-edit, after typing 3 2 2 2 the clock might look like this:
To finish the edit, press ↵ or Tab. To exit an edit without changing the clock press ESC. If you mis-type an entry so that the new value would be illegal (for example, resulting in more than 30 frames when Timecode is set to 30 frames per second), the clock will reset at the end of the edit, and move the cursor back to the start so that you can start over.
There is a shortcut available for those who wish to be able to edit the transport clocks entirely without the mouse. It can be found in Window > Key Bindings > Transport > Focus On Clock. If bound to a key (÷ on the numerical keypad is the default), then pressing that key is equivalent to clicking on the primary (left) transport clock, and editing can begin immediately.
One detail of the editing design that is not immediately obvious is that it is possible to enter part of a full time value. Suppose that the clock is in BBT mode, displaying 024|03|0029, and you want to alter the value to the first beat of the current bar. Click on the clock and type 0 1 0 0 0 0. Similarly, if it is in Minutes:Seconds mode, displaying 02:03:04.456, and you want to get to exactly 2 hours, click on the clock and type 0 0 0 0 0 0 0 to reset the minutes, seconds and milliseconds fields.
You can also type values into the clock that are intended as a relative change, rather than a new absolute value. Simply end the edit by pressing + or - (the ones on any keypad will also work). The plus key will add the entered value to the current value of the clock, minus will subtract it. For example, if the clock is in Samples mode and displays 2917839, you move it back 2000 samples by typing 2 0 0 0 and -, rather than ending with Enter or Tab.
Position the mouse pointer over the clock, and move the scroll wheel. Moving the scroll wheel up (⇑) increases the value shown on the clock, moving it down (⇑) decreases it. The step size is equal to the unit of the field you are hovering over (seconds, hours, etc.).
Position the mouse pointer over the clock, press the left mouse button and drag. Dragging upwards increases the value shown on the clock, dragging downwards decreases it, again with a step size equal to the unit of the field you began the drag on.