DateTimeUtils - Helper logic to handle time

[rmeissner]

🧐 Motivation
Time is important and off-chain there are often cases where time related checks need to be performed. To handle this on chain it would be nice to have some utils that would encapsulate these checks.

There are many use cases for this:

• Assuming we have a multisignature wallet that should only require the approval of all owners if more than a specified amount should be transferred (see https://github.com/gnosis/MultiSigWallet/blob/master/contracts/MultiSigWalletWithDailyLimit.sol). In the linked contract this limit is always the "in the last 24h" but it would be interesting to map this to a calendar day.
• Assuming we want to perform recurring transfer (see https://groundhog.network/) it makes sense to limit these to calendar dates (On the 1st of November I pay my monthly zeppelin subscription, or on the 13th of November between 1pm and 2pm the company pays me my salary).

📝 Details
There is a library that is already implementing this logic (https://github.com/pipermerriam/ethereum-datetime), but this is not really up to date, nor audited.

Also it would be interesting to consider edge cases like leap seconds (at least mention why they are not implemented).

An api should include method to check if what year/ month/ day/ hour/ minute a certain timestamp is.

[nventuro]

A point we'll have to address if attempting to cater to a 'calendar day' usage will be that of timezones, which may be a major issue due to daylight savings, etc.

[rmeissner]

Yes that is true. I would probably assume UTC for simplicity and it would be possible to shift the time (e.g UTC+1 or UTC-10)

[frangio]

Leap seconds are dealt with at the EVM level. The unix timestamp that is obtained from block.timestamp doesn't count real seconds, it counts "civil" seconds. Notice in this table how the same timestamp ending in 800 actually spans two real seconds.

[nventuro]

In #533, the case was made for extending this to also include support for a cleaner Date type. According to Solidity developers here, this should be done by us, not the language.

[alcuadrado]

#533 is quite old, and there's some discussion in that issue related to the security/correctness of using block.timestamp. I think that discussion is not relevant anymore, but the original proposal by @martriay is still relevant.

There's some ambiguity in #533, as it is not clear if it proposes Date to be a function or a type because of the capital D. I think it was intended to be a function, which IMO would be a great starting point.

[alcuadrado]

The reason I commented on this issue is that while auditing a smart contract I found errors in the way dates were computed.

The contract I was auditing had to lock some tokens until a certain date. It computed dates with now + offset. This has two problems: (1) it depends on the deployment date. (2) the offset is easy to get wrong (spoiler: it was wrong).

I'd love to recommend the authors to use a human-readable representation of the dates, but I couldn't find a maintained function/lib for doing so. Having something like dateToTimestamp(year, month, day) would be enough for this use-case.

[frangio]

So, functions to create timestamps from human dates: date(2019, 06, 12), datetime(2019, 06, 12, 00, 00). Sounds quite reasonable and useful.

Would these functions return the uint256 timestamp directly, or a struct with operations?

Also, how should we deal with timezones?

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(2) the offset is easy to get wrong (spoiler: it was wrong).

@alcuadrado Mind sharing how it was wrong exactly?

[nventuro]

Timezones are a mess, and DST only complicates matters further. I'd make it so that times specified in either UTC, or at a fixed offset (e.g. UTC+8 for Hong Kong, UTC-4 for Chile, etc.).