Richard Suchenwirth 2004-09-13 - http://www.faqs.org/faqs/astronomy/faq/part3/ (item C.11) describes John Horton Conway's approximation for the moon phase of a given date. Here it is in Tcl:
proc moonphase date {
set y [clock format $date -format %Y]
set delta [expr {$y/100==19? 4: 8.3}]
scan [clock format $date -format %m] %d m
set m [expr {$m==1? 3: $m==2? 4: $m}]
scan [clock format $date -format %d] %d d
set t [expr ($y%100)%19]
if {$t>9} {incr t -19}
puts y:$y,delta:$delta,m:$m,d:$d,t:$t
expr {(round(($t*11)%30+$m+$d-$delta)%30)}
}Testing:
% moonphase [clock scan "June 6, 1944"] 14
which matches the FAQ's example and stands for (almost) full moon.