Can you explain the difference between US GAAP and IFRS in the calculation of the interest cost of a defined benefit pension plan?

For US GAAP interest cost (which we see in the reconciliation of PBO) is the same as the interest cost we see in I/S. Both use opening PBO x r.

However for IFRS these two are NOT the same.  The reconciliation of DBO uses the same approach as US GAAP but the I/S value is not the same. Instead for the I/S we have opening funded status x r.

Very important distinction.