Problem With IR Timer/ LCD Project

I'm new to Arduino development so forgive any stupid questions please!

I've used this tutorial successfully to get the LCD working. All is fine.

I want to set up a Timer for a science project using two IR LED boards. I came across this project and am trying to display the time on the LCD display:

I've had to change the pins in this project to Digital Pin6 and Pin7 (from Digital Pin2 and Pin3). Pin13 is free as per the project description.

My circuit is wired up pretty much as the Hello World LCD project +:
My 1st IR sensor Vin: connected to 5v on board, GND: connected to GND on baord, OUT is connected to D6 (the LED flashes and D6 changes from 1 to 0 when an object passes in front of it)
My 2nd IR sensor is the same except OUT is connected to D7 (the LED also flashes and D7 changes from 1 to 0 when an object passes in front of it)

It seems that t1 and t2 never seem to get set so my time isn't calculated. When I watch the state of D6 and D7 on the serial monitor, they change from 1 to 0 when something passes in front of them.
Any ideas where I've gone wrong?

Here is the code (I'm just using the Serial Monitor to try and figure out rather than the LCD which is still wired up):

// reads two IR proximity/line-detectors. reports time between triggers.
// uses interrupts. First one should go high for detect on 2
// second detector should go high on detect on pin 3

unsigned long t1=0;
unsigned long t2=0;
void sens1() { if ((t1==0) && (t2==0)) t1=micros(); }
void sens2() { if ((t2==0) && (t1!=0)) t2=micros(); }

void setup() {

void loop() {

if (t2>0)

the attachInterrupt() function only works for INT0 and INT1 which are part of pins 2 and 3. You can not use them for pins 6 and 7. (Assuming you are using a Uno or Nano)

Hi bhl64, that's exactly it! Thanks so much for your help!