The times printed on an eticket I bought match with what ernestnywang posted, which is 2050/2315 & 0010/0630. The press releases given earlier have other times listed.

However, I don't really understand why the block time for HKG-CCU is a full 1 hour 5 minutes longer than CCU-HKG.

Based on the distance wise, I think the block time for CCU-HKG is more sensible, because the distance is similar to HKG-SIN, where the block times are also 4 hours or less.

If I have to guess, actual flight time should be just over three and a half hours each way.

It could be that KA is worried that about the following:
1) Too many incoming feeder flights are late causing delays to onward flight out of HKG (I've seen many of these type of delays on other India flights)
2) There may be ATC delays upon arrival into CCU airspace and extended holding will be required often
3) Or, ...someone messed up