Daylight duration symmetry between the northern and southern hemispheres is a fundamental astronomical fact arising from Earth's axial tilt and geometry, requiring no citation.
The claim is a direct consequence of Earth's geometry and axial tilt: at any given time, the total daylight hours experienced globally are split complementarily between hemispheres, and across the year each hemisphere experiences the exact same annual total and seasonal mirror images due to orbital mechanics. This is a matter of basic astronomical definition and everyday observation, making it common knowledge.