The shell command ignores anything else on the gnuplot command line. If instead you want to pass a command string to a shell for immediate execution, use the system function or the shortcut !. See system.
shell system "print previous_plot.ps" ! print previous_plot.ps current_time = system("date")
Copyright 1986 - 1993, 1998, 2004 Thomas Williams, Colin Kelley
Distributed under the gnuplot license (rights to distribute modified versions are withheld).