Here's how I get data into Excel columns, for experiments:
- Serial.print() what I want to see, with a convenient delimiting character between data elements. A comma or a space usually works fine.
- Copy and paste it into a single Excel column.
- Data > Text to columns > Delimited > describe the delimiter
And, voila, it's in columns. For your application, you'll just want to be sure that your delimiter isn't a character that will show up in the timestamp you add later.