Tax receipts will be issued for the current year for all donations received by 11:59 pm Eastern Time on December 31st.
charity: water operates on Eastern Time, so all donations and tax receipts are dated accordingly. However, if you are in a different time zone, the email itself which contains your tax receipt should be time-stamped according to your location.
All mailed-in checks must be postmarked by December 31st.
If your donation is received after the deadline, you'll receive a tax credit for the subsequent year instead.