When the disciples warn Jesus about returning to Judea, he answers, 'Are there not twelve hours in the day?' and speaks of walking while there is light rather than stumbling in darkness.
The saying begins with an ordinary way of reckoning daylight and develops it metaphorically.
In Jewish and wider Greco-Roman practice, daylight could be divided into twelve hours from sunrise to sunset. Because daylight length changes with the seasons, these were not always sixty-minute hours by modern clock measurement. The sixth hour was around midday and the twelfth near sunset, with variation according to season and latitude.
Jesus uses this familiar structure to speak about an allotted period in which one's work can be carried out.
The timekeeping background keeps the image intelligible. Jesus is not making a scientific claim that daylight is always exactly twelve modern hours.
Within the narrative, the metaphor expresses purposeful movement under the Father's timing: danger does not cause Jesus to abandon the work appointed for his 'day.'