Per IRS Publication 521 Moving Expenses, page 6:

Time Test Not Yet Met
You can deduct your moving expenses on your 2017 tax return even though you haven’t met the time test by the date your 2017 return is due. You can do this if you expect to meet the 39-week test in 2018 or the 78-week test in 2018 or 2019.

If you don’t deduct your moving expenses on your 2017 return, and you later meet the time test, you can file an amended return for 2017 to take the deduction. See When To Deduct Expenses, later, for more details.

Failure to meet the time test. If you deduct moving expenses but don’t meet the time test in 2018 or 2019, you must either:

1. Report your moving expense deduction as other income on your Form 1040 for the year you can’t meet the test, or
2. Use Form 1040X to amend your 2017 return, figuring your tax without the moving expense deduction.