People do not confuse night and day, even when they don't know what time it is. Similarly, when the time of day is close to dawn or dusk, you know... The investigation into this question delves into witness checking - to confirm testimony, to validate their reliability - with time as one of the means of checking accuracy. How much leeway do we need to establish a specific hour (in the absence of clocks)? Also - How much discrepancy is acceptable between witnesses? And what about chametz times? Why isn't there the same leeway. Plus, the Gemara notes how different are the component parts of this comparison, including the fuzziness of our experience of time.