This is a simple tool written in Ruby 1.8 that will allow you to add a delay (positive or negative) to a Substation Alpha (ASS/SSA) subtitle file. At present, this is the only format supported.
Set the script as executable before first use.
./suboffset.rb inputfile.ass outputfile.ass delay
Where delay is expressed in seconds as a signed decimal, e.g., 0.21 or -1.23
Place the script in ~/bin for ease of use if desired.