You can use negative times in TENT or CSPLIN -- this is allowed for just the situation you are in. You might want to do TENTzero(-12,12,13), which should give one "knot" (break-point for the TENT) every 2 seconds, with the "zero" meaning that the first and last points in the TENT-modeled HRF will be set to zero -- thus you will have 11 basis functions.