tick.sh

#!/bin/bash

A=$(date +%Y-%m-%dT%H-%M-%S)
B=`date +%Y-%m-%dT%H-%M-%S`

while true; do
  echo $A
  echo $B
  sleep 2
done

I made the modifications as I had suggested:

sdake@beast-01:~/go/src/istio.io/test-infra$ more jwen.sh 
#!/bin/bash

A=$(date +%Y-%m-%dT%H-%M-%S)
B=$(date +%Y-%m-%dT%H-%M-%S)

while true; do
  echo A=$A
  sleep 2
  echo B=$B
done

and the result is

sdake@beast-01:~/go/src/istio.io/test-infra$ ./jwen.sh
A=2020-02-05T06-12-15
B=2020-02-05T06-12-15
A=2020-02-05T06-12-15
B=2020-02-05T06-12-15
A=2020-02-05T06-12-15
B=2020-02-05T06-12-15
A=2020-02-05T06-12-15
B=2020-02-05T06-12-15
A=2020-02-05T06-12-15
B=2020-02-05T06-12-15
A=2020-02-05T06-12-15
B=2020-02-05T06-12-15
A=2020-02-05T06-12-15
B=2020-02-05T06-12-15
A=2020-02-05T06-12-15

which looks like $(...) is working correctly...