When I compile the program and load and run it with

  (asdf:oos 'asdf:load-op :clx)
  (load "package")
  (load "clock")
  (st-clock:run)

it either completely crashes SBCL or it freezes (i.e. the clock stops)
sooner or later, usually within the first 24 hours of its run.

It looks like a SBCL bug to me.

In case of crash, the crash is reported as follows:

  Argh! gc_find_free_space failed (first_page), nbytes=16.
     Gen Boxed Unboxed LB   LUB  !move  Alloc  Waste   Trig    WP  GCs Mem-age
     0: 129350     0     0     0     0 529397048 420552  2000000    0   0  0.0000
     1:  1267   399    42    14   264  6942168 111144  8757120 1039   1  3.9396
     2:     0     0     0     0     0        0     0  2000000    0   0  0.0000
     3:     0     0     0     0     0        0     0  2000000    0   0  0.0000
     4:     0     0     0     0     0        0     0  2000000    0   0  0.0000
     5:     0     0     0     0     0        0     0  2000000    0   0  0.0000
     6:     0     0     0     0     0        0     0        0    0   0  0.0000
     Total bytes allocated=536339216
  fatal error encountered in SBCL pid 15650
  The system is too badly corrupted or confused to continue at the Lisp
  level. If the system had been compiled with the SB-LDB feature, we'd drop
  into the LDB low-level debugger now. But there's no LDB in this build, so
  we can't really do anything but just exit, sorry.

In case of freeze, gdb indicates the freeze happens at the following
place:

  0x0805864e in futex_wait (lock_word=0x986c514, oldval=-1213677184)
      at linux-os.c:64
  64      _syscall4(int,sys_futex,
  (gdb) where
  #0  0x0805864e in futex_wait (lock_word=0x986c514, oldval=-1213677184)
      at linux-os.c:64

Lisp backtrace of the freeze after pressing Ctrl-C:

  debugger invoked on a SIMPLE-CONDITION in thread 28863: interrupted at #X805861E

  You can type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.

  restarts (invokable by number or by possibly-abbreviated name):
    0: [CONTINUE] Return from SB-UNIX:SIGINT.
    1: [ABORT   ] Reduce debugger level (leaving debugger, returning to toplevel).
    2: [TOPLEVEL] Restart at toplevel READ/EVAL/PRINT loop.
  ("foreign function: call_into_lisp")
  0] ba

  0: ("foreign function: call_into_lisp")
  1: ("foreign function: funcall3")
  2: ("foreign function: interrupt_handle_now")
  3: ("foreign function: #x80531F5")
  4: ("bogus stack frame")
  5: ("hairy arg processor for top level local call SB!THREAD::GET-MUTEX/FUTEX"
      #<unavailable argument>
      #<unavailable argument>
      #<unavailable argument>)
  6: (XLIB::START-PENDING-COMMAND #<unavailable argument>)
  7: (XLIB::GET-DRAWABLE-GEOMETRY-BUFFER #<unavailable argument>)
  8: (XLIB:DRAWABLE-HEIGHT #<unavailable argument>)
  9: (SPRINGTAIL-CLOCK::REDRAW
      #<XLIB:WINDOW :0 56623106>
      #<XLIB:GCONTEXT :0 56623107>
      #<XLIB:FONT fixed :0 56623105>
      3313931441)
  10: (SPRINGTAIL-CLOCK::CLOCK
       #<XLIB:DISPLAY :0 (The XFree86 Project, Inc R40300001)>
       #<XLIB:WINDOW :0 56623106>
       #<XLIB:GCONTEXT :0 56623107>
       #<XLIB:FONT fixed :0 56623105>)
  11: ("XEP for SPRINGTAIL-CLOCK::CLOCK"
       #<unavailable argument>
       #<unavailable argument>
       #<unavailable argument>
       #<unavailable argument>
       #<unavailable argument>)[:EXTERNAL]
  12: (SPRINGTAIL-CLOCK:RUN)
  13: ("XEP for SPRINGTAIL-CLOCK:RUN" #<unavailable argument>)[:EXTERNAL]
  14: (SB-INT:EVAL-IN-LEXENV
       2
       (SPRINGTAIL-CLOCK:RUN)
       #S(SB-KERNEL:LEXENV
          :FUNS NIL
          :VARS NIL
          :BLOCKS NIL
          :TAGS NIL
          :TYPE-RESTRICTIONS NIL
          :LAMBDA NIL
          :CLEANUP NIL
          :HANDLED-CONDITIONS NIL
          :DISABLED-PACKAGE-LOCKS NIL
          :POLICY ((SAFETY . 3) (COMPILATION-SPEED . 2) (DEBUG . 2) (SPEED . 1)
                   (SPACE . 1) (INHIBIT-WARNINGS . 1))))[:EXTERNAL]
  15: (INTERACTIVE-EVAL 1 (SPRINGTAIL-CLOCK:RUN))[:EXTERNAL]
  16: (SB-IMPL::REPL-FUN 1 NIL)[:EXTERNAL]
  17: ("#'(LAMBDA NIL (HANDLER-BIND # #))")
  18: ("XEP for #'(LAMBDA NIL (HANDLER-BIND # #))" 0)[:EXTERNAL]
  19: (SB-IMPL::%WITH-REBOUND-IO-SYNTAX 1 #<FUNCTION "CLOSURE" {90A734D}>)[:EXTERNAL]
  20: (SB-IMPL::TOPLEVEL-REPL 1 NIL)[:EXTERNAL]
  21: (SB-IMPL::TOPLEVEL-INIT 0)[:EXTERNAL]
  22: ("FLET SB!IMPL::RESTART-LISP")

  0]