None.
Note:
Your use of millis() is slightly wrong. You should never use addition, you should use subtraction. The human brain does not have a problem with it but a microcontroller works differently.
Not solving your problem but change
if (millis() > t + serialPrintInterval) {
to
if (millis() -t > serialPrintInterval) {
PS
You might benefit from reading Robin's updated Serial Input Basics tutorial.