Tax receipts will be issued for the current year for all donations received by 11:59 pm Eastern Standard Time on December 31st.
charity: water is located on the East Coast and operates on EST, 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.