Although the contribution would not need to be cashed in 2017, the postmark would need to read 12/31/17.
Excerpt from link below:
'Pay attention to the calendar. Contributions are tax-deductible in the year the contributions are made. To make it count during the tax year, gifts must be made by December 31. That doesn't necessarily mean cash out of your account. Contributions made by text message are deductible in the year you send the text message if the contribution is charged to your telephone or wireless account. Credit card charges - even if they're not paid off before the end of the year - are deductible so long as the charge is captured by year end. Similarly, checks which are written and mailed by the end of the year will be deductible for this year even if they aren’t cashed in 2017. Good intentions don't count: making announcements that you intend to donate assets will not qualify for a deduction in the current tax year unless you make good on the pledge during the year.'
https://www.forbes.com/sites/kellyphillipserb/2017/12/11/13-tips-for-making-your-charitable-donation...
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