when offsetting keys in graph editor, the commands are as follows

keyframe -e -iub true -r -o over -tc 89.304 -t 20.696 casual21_m_mediumpoly:Bip01_R_Toe0_rotateZ ; ( eg )

need to recreate this (python ) using script, it does not work. Problem is with the -t parameter. Question is

1) how is the t value derived

2) it is observed that in some cases the t parameter is set and in some cases it is not. ??