I reckon the 'reset' button simply clears the input you typed into the TC's. So if you typed '13212009' and then realise 'oh yeah, that means the 13th month, which is impossible' (for example) then you could tap the reset button and it would just revert to the previous date. Although, Blue et al have decided on a strictly canon mod, so if the button is not used in the film, they're probably not going to include it unless some evidence can be found to suggest its function.