You can make contributions to your SEP-IRA up to April 17, 2018 and deduct them on your 2017 tax return.

For contributions made between January 1, 2018 and April 17, 2018, make sure to indicate to the SEP-IRA administrator that they are contributions for tax year 2017.

For SEP contributions, it's the responsibility of the employer (or self-employed individual), not the IRA custodian, to track the year for which contributions are made;  the Form 5498 prepared by the custodian only reports the year *in* which the contribution was made, not the year *for* which the contribution was made.  Just make sure that the custodian knows its a SEP contribution (for a contribution to an account that was established as a SEP-IRA, the custodian will designated as a SEP contribution by default) so that it appears on the Form 5498 as a SEP contribution.