There is a need to focus on the accuracy of the time that is attached
to solar radiation measurements. In this paper the effect of time erro
rs on the calculation of the clearness index is investigated. An appro
ximate expression for the error in clearness index due to time errors
is derived. Using instantaneous solar radiation measurements from Town
sville, Australia, the effect of time errors is illustrated with cumul
ative probability distribution curves. It is shown that even relativel
y small time errors (10 min) can substantially change the distribution
curves. Time errors can be caused by drifts or inaccuracies of the da
ta recording device's clock.