We present timing analysis results for Rossi x-ray Timing Explorer (RXTE) observations of x-ray binary source 4U 1820-30
located in the globular cluster NGC 6624. The light curves of observations made between October 1996 and September 1997
show that the maximum of the 685s binary period modulation folded by the linear ephemeris from previous observations has
phase shift of -0.20 ± 0.06. Combined with historical results (1976-1997), the binary period derivative is measured
to be dP / P = (-3.47 ± 1.48) x 10-8 yr-1. The previous known (~176d) long-term modulation is
significant in the x-ray light curve obtained by analysis of the RXTE
All-Sky Monitor (ASM) during the years 1996-2000. The RXTE/ASM ephemeris is extended by analysis of all historical data
(Vela 5B and Ginga) to yield a period 171.033 ± 0.326 days with no evidence for period change
(|dP/ P| < 2.20 x 10-4 yr-1). All reported X-ray burst activity is confined to within ± 23d
of the predicted minima. This stable long-term modulation is consistent with 4U 1820-30 being a hierarchical triple
system with a ~1.1d period companion.