Charting help needed

Here is a formula you can use:

=IFERROR((A2-XLOOKUP(G2,G$1:G1,A$1:A1,NA(),0,-1))*86400000,"")

In the image below you can see it highlighted in column I

NOTE: I used column A and multiplied by 864e5 to convert from time difference to ms so that will get rid of those really large negative numbers each time the SFN rolls over (see row 27 in the image) but later in the data there is a time gap/jump that results in a very large number but that is now more 'wrong' than just using the SFN which had rolled over multiple times in that span.  If you want the formula could even 'screen' those values out.  But if you don't like using the actual time difference then just shift those column A references back to column E.

Also: a possible 'easy' option would be to just SORT the data by RNTI so that all the same RNTI data is grouped together so your simple formula would work (except for the transition point(s) ).