How fast does the clock of Finance run? - A time-definition enforcing   scale invariance and quantifying overnights
  A symmetry-guided definition of time may enhance and simplify the analysis of historical series with recurrent patterns and seasonalities. By enforcing simple-scaling and stationarity of the distributions of returns, we identify a successful protocol of time definition in Finance. The essential structure of the stochastic process underlying the series can thus be analyzed within a most parsimonious symmetry scheme in which multiscaling is reduced in the quest of a time scale additive and independent of moment-order in the distribution of returns. At the same time, duration of periods in which markets remain inactive are properly quantified by the novel clock, and the corresponding (e.g., overnight) returns are consistently taken into account for financial applications. 
Michele Caraglio, Fulvio Baldovin and Attilio L. Stella
q-fin.ST cond-mat.stat-mech physics.data-an q-fin.CP