Support timestamps

[DJRHails]

The standard library should include some support for timestamps. Although difficult and dangerous in decentralised environments a large segment of contracts use them extensively in relatively safe ways. In Flint we would want the safe way to be the only way.

Using the block timestamp is dangerous as it has been shown that it can be manipulated by a miner. However this, through game theory principles, is limited in scope.

Two suggested methods for including time dependence in a safe way are:

Restricted to fixed timescales

Due to the reverse incentive for other miners to mine on top of a block clearly in the future. 12 Minutes has been approximated as a safe dift boundary.

if startTime.isNHoursLater(2) {

}

struct Time {
  functions isNHoursLater(hour: Int) {
    return blockTimestamp() > self.start + hour * HOUR_FACTOR
  }
}

Block based timestamp

Block numbers and average block time can be used to estimate time, but this is not future proof as block times may change (such as the changes expected during Casper).

Namely a new block is mined approximately every 12s therefore we can include a standard library function like below:

if Time.roughlyExceeds("2018-07-01", "19:00") {

}

struct Time {
  functions roughlyExceeds(date: Date, time: Time) {
    epocTime = epoc(date, time) // Number of seconds from first block creation
    expectedBlockNumber = epoc / 12 
    return blockNumber() > expectedBlockNumber
  }
}