How to convert posixTime to slot number on Cardano Testnet

Question

I'd like to obtain the slot number corresponding to a posixtime on Cardano Testnet.

For example, this might be useful if you need to set a slot number in cardano-cli, say building a transaction: cardano-cli transaction build --invalid-before slotNumber

Answer 1

I share this code snippet with the converter.

module Week03.Converter ( posixTimeToSlotTestnetConverter ) where 

import Ledger          ( POSIXTime(POSIXTime), Slot(Slot) )
import Ledger.TimeSlot ( SlotConfig(SlotConfig), posixTimeToEnclosingSlot )

posixTimeToSlotTestnetConverter :: POSIXTime -> Slot
posixTimeToSlotTestnetConverter time = slotWhenSlotChangedTo1Sec + posixTimeToEnclosingSlot testnetConf time

timeWhenSlotChangedTo1Sec :: POSIXTime
timeWhenSlotChangedTo1Sec = POSIXTime 1595967616000  -- 2020/07/28 20:20:16 - epoch:74 - slot:1598400 - block:1597133  

slotWhenSlotChangedTo1Sec :: Slot
slotWhenSlotChangedTo1Sec = Slot 1598400

testnetConf :: SlotConfig
testnetConf = SlotConfig 1000 timeWhenSlotChangedTo1Sec

Kudos to @Travis for timeWhenSlotChangedTo1Sec value. See: When was the slot duration set to 1 sec on Cardano Testnet?

Answer 2

The accepted answer already explains how the calculation must be made and provides a sample code in Haskell; however, I will also leave here a simple snippet of code in Java, removing all the Haskell boilerplate (and can be easily ported to any other language).

/**
 * Converts from POSIX time to slot.
 *
 * @param time The POSIX time to be converted (in seconds).
 *
 * @return The matching slot.
 */
static long posixTimeToSlot(long time)
{
    long slotWhenSlotChangedTo1Sec = 1598400;
    long timeWhenSlotChangedTo1Sec = 1595967616L;

    long remainingSeconds = time - timeWhenSlotChangedTo1Sec;

    return slotWhenSlotChangedTo1Sec + remainingSeconds;
}