I have someone looking to contribute to a 401k instead of a sep for 2017. However, some was already contributed to the sep.

1) Can we correct the contributions by taking them out of the sep? If so, what is the deadline?

2) I understand the employer contribution deadline for a solo 401k is the tax filing deadline. Are their contributions as an employee carrying a deadline of the tax filing date also or must they be made within the 2017 calendar year?