  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 $\pm$ 0.06. Combined with historical results (1976-1997), the binary
period derivative is measured to be $\dot P / P =(-3.47 \pm 1.48) \times
10^{-8}$ yr$^{-1}$. The previous known ($\sim$176 d) long-term modulation is
significant in the x-ray light curve obtained by analysis of the RXTE All-Sky
Monitor (ASM) during the years 1996-2000. The RXTE/ASM ephemeris is extended by
analysis of all historical data (Vela 5B and Ginga) to yield a period
171.033$\pm$0.326 days with no evidence for period change ($|{\dot{P}/ {P}}| <
2.20 \times 10^{-4}$ yr$^{-1}$). All reported x-ray burst activity is confined
to within $\pm$23 d of the predicted minima. This stable long-term modulation
is consistent with 4U 1820$-$30 being a hierarchical triple system with a
$\sim$1.1 d period companion.
