Alright, I have this code I'm modifying from here and I modified to to print outputs from two other sensors and execute a relay after a bit. When I tried to implement the capability to write this data to an SD card the data stopped being printed. I think I've narrowed the problem to the CS pin for the Tentacle Shield also being used by the SD card and is therefor not being initialized. Also, when the line "if (!SD.begin(4)) {" is included, the value next_serial_time overflows to 2^32 -1. Here's my code.
Please post your code using code tags, or if it is too large, as an attachment. Most of us don't trust external links.