Here is the statement:
"IF(F13/F121,IF(I130,IF(C120,I13-C12,IF(C100,I13-C10,IF(C90,I13-C9,IF(C80,I13-C8,IF(C70,I13-C7,IF(C60,I13-C6,IF(C50,I12-C5,0))))))),0),I13-C13)"

I have too many statements.

Column C will have a time stamp. One time stamp in column C could apply to
several rows of data. The row count starts with 1, and is counted in column
F. Each row of data in Column I must reference the respective time stamp in
column C, and ignore all null values. Basically, if it's blank, it needs to
refer to the prior cell, until a time stamp is found. This is eating a lot of
IF statements.

Column I is the a secondary time stamp - if a cell in Column I is blank, the
corresponding value in Column H should be 0. This is working fine.

I have about 30 rows, and need to account for the possibility of only one
time stamp. Meanwhile, if other time stamps occur, they should be referenced,
instead of the first..

Is this making sense?