Use zsh for bin/since

Why depend on bc for this, when we know zsh will always be available on
my nix systems.

bin/since

@@ -1,5 +1,4 @@
-#!/usr/bin/env cached-nix-shell
-#! nix-shell -i bash -p bc
+#!/usr/bin/env zsh
 set -e
 (( $# > 0 )) || exit
 
@@ -25,23 +24,15 @@ fi
 
 # Bash doesn't support floating point arithmetic like zsh does, so use bc to do
 # our arithmetic.
-bc <<EOF
-  define abs(i) {
-    if (i < 0) return (-i)
-    return (i)
-  }
-
-  t = abs($t)
-
-  weeks = t / 60 / 60 / 24 / 7
-  days = t / 60 / 60 / 24 % 7
-  hours = t / 60 / 60 % 24
-  minutes = t / 60 % 60
-  seconds = t % 60
-
-  if (weeks > 0) print weeks, "w "
-  if (days > 0) print days, "d "
-  if (hours > 0) print hours, "h "
-  if (minutes > 0) print minutes, "m "
-  print seconds, "s"
-EOF
+zmodload zsh/mathfunc
+t=$(( abs(t) ))
+weeks=$(( t / 60 / 60 / 24 / 7 ))
+days=$(( t / 60 / 60 / 24 % 7 ))
+hours=$(( t / 60 / 60 % 24 ))
+minutes=$(( t / 60 % 60 ))
+seconds=$(( t % 60 ))
+(( weeks > 0 )) && echo -n "${weeks}w "
+(( days > 0 )) && echo -n "${days}d "
+(( hours > 0 )) && echo -n "${hours}h "
+(( minutes > 0 )) && echo -n "${minutes}m "
+echo -n "${seconds}s"