Index: conversions.rb
===================================================================
--- conversions.rb	(revision 280)
+++ conversions.rb	(revision 281)
@@ -24,13 +24,13 @@
         def to_hours
           s = self.dup
           s.strip!
-          if s =~ %r{^(\d+([.,]\d+)?)h?$}
+          if s =~ %r{^([-]?\d+([.,]\d+)?)h?$}
             s = $1
           else
             # 2:30 => 2.5
-            s.gsub!(%r{^(\d+):(\d+)$}) { $1.to_i + $2.to_i / 60.0 }
+            s.gsub!(%r{^([-]?\d+):(\d+)$}) { $1.to_i + $2.to_i / 60.0 }
             # 2h30, 2h, 30m => 2.5, 2, 0.5
-            s.gsub!(%r{^((\d+)\s*(h|hours?))?\s*((\d+)\s*(m|min)?)?$}) { |m| ($1 || $4) ? ($2.to_i + $5.to_i / 60.0) : m[0] }
+            s.gsub!(%r{^(([-]?\d+)\s*(h|hours?))?\s*((\d+)\s*(m|min)?)?$}) { |m| ($1 || $4) ? ($2.to_i + $5.to_i / 60.0) : m[0] }
           end
           # 2,5 => 2.5
           s.gsub!(',', '.')