On May 9 the ZenCash team sent an email with the subject line:
Big Change! ZenCash Fork at Zclassic Block 110,000 on May 16
The email listed a few questions with answers, among them:
Question: When will block 110,000 happen?
Answer: Should be sometime on Tuesday May 16. There is usually 24 blocks per hour.
This means about (24*24) 576 blocks per day.
Right now the Zclassic block explorer shows a height of 108591, there are about 1409 blocks waiting to be mined. So, this should be pushing the fork date by a couple of days. Probably May 18.
Edit: Bittrex had the right calculation
Bittrex users will be credited 1 ZEN for each Zclassic (ZCL) held in their exchange accounts at Zclassic block 110,000. The block is estimated to occur at approximately 11:10 PDT (18:10 UTC) 5/18/2017.
Support for the ZEN Hardfork