[Ethereum] Is block.timestamp safe for longer time periods

If you are using geth here's a patch that I use to accelerate mining. You could probably modify it to decelerate mining if you wish. (I'm curious why you wish to go slower). In any event, I think the answer to "how does the interval for mining get set" is "not very clear from the code"... but this patch should give you a head start in understanding the timing of geth.

diff --git a/eth/fetcher/fetcher.go b/eth/fetcher/fetcher.go
index d88d919..f03fd30 100644
--- a/eth/fetcher/fetcher.go
+++ b/eth/fetcher/fetcher.go
@@ -34,7 +34,7 @@ import (
 const (
    arriveTimeout = 500 * time.Millisecond // Time allowance before an announced block is explicitlrd
    gatherSlack   = 100 * time.Millisecond // Interval used to collate almost-expired announces witfs
-   fetchTimeout  = 5 * time.Second        // Maximum alloted time to return an explicitly requestebk
+   fetchTimeout  = 1 * time.Second        // Maximum alloted time to return an explicitly requestebk
    maxUncleDist  = 7                      // Maximum allowed backward distance from the chain head
    maxQueueDist  = 32                     // Maximum allowed distance from the chain head to queue
    hashLimit     = 256                    // Maximum number of unique blocks a peer may have annoued
diff --git a/eth/peer.go b/eth/peer.go
index 15ba22f..0201057 100644
--- a/eth/peer.go
+++ b/eth/peer.go
@@ -41,7 +41,7 @@ var (
 const (
    maxKnownTxs      = 32768 // Maximum transactions hashes to keep in the known list (prevent DOS)
    maxKnownBlocks   = 1024  // Maximum block hashes to keep in the known list (prevent DOS)
-   handshakeTimeout = 5 * time.Second
+   handshakeTimeout = 1 * time.Second
 )

 // PeerInfo represents a short summary of the Ethereum sub-protocol metadata known
diff --git a/miner/worker.go b/miner/worker.go
index 754a6fc..2b62b59 100644
--- a/miner/worker.go
+++ b/miner/worker.go
@@ -466,7 +466,7 @@ func (self *worker) commitNewWork() {
            tstamp = parent.Time().Int64() + 1
    }
    // this will ensure we're not going off too far in the future
-   if now := time.Now().Unix(); tstamp > now+4 {
+   if now := time.Now().Unix(); tstamp > now {
            wait := time.Duration(tstamp-now) * time.Second
            glog.V(logger.Info).Infoln("We are too far in the future. Waiting for", wait)
            time.Sleep(wait)
diff --git a/p2p/rlpx.go b/p2p/rlpx.go
index aaa7338..ce82eb7 100644
--- a/p2p/rlpx.go
+++ b/p2p/rlpx.go
@@ -57,7 +57,7 @@ const (

    // total timeout for encryption handshake and protocol
    // handshake in both directions.
-   handshakeTimeout = 5 * time.Second
+   handshakeTimeout = 2 * time.Second

    // This is the timeout for sending the disconnect reason.
    // This is shorter than the usual timeout because we don't want
diff --git a/params/protocol_params.go b/params/protocol_params.go
index dcc17e0..21a49c5 100755
--- a/params/protocol_params.go
+++ b/params/protocol_params.go
@@ -29,10 +29,10 @@ var (
    CallNewAccountGas      = big.NewInt(25000)  // Paid for CALL when the destination address didn'e.
    TxGas                  = big.NewInt(21000)  // Per transaction. NOTE: Not payable on data of cas.
    TxDataZeroGas          = big.NewInt(4)      // Per byte of data attached to a transaction that u.
-   DifficultyBoundDivisor = big.NewInt(2048)   // The bound divisor of the difficulty, used in thep.
+   DifficultyBoundDivisor = big.NewInt(1)      // The bound divisor of the difficulty, used in thep.
    QuadCoeffDiv           = big.NewInt(512)    // Divisor for the quadratic particle of the memoryo.
-   GenesisDifficulty      = big.NewInt(131072) // Difficulty of the Genesis block.
-   DurationLimit          = big.NewInt(13)     // The decision boundary on the blocktime duration e.
+   GenesisDifficulty      = big.NewInt(1)      // Difficulty of the Genesis block.
+   DurationLimit          = big.NewInt(1)      // The decision boundary on the blocktime duration e.
    SstoreSetGas           = big.NewInt(20000)  // Once per SLOAD operation.
    LogDataGas             = big.NewInt(8)      // Per byte in a LOG* operation's data.
    CallStipend            = big.NewInt(2300)   // Free gas given at beginning of call.
@@ -57,7 +57,7 @@ var (
    CreateDataGas        = big.NewInt(200)    //
    Ripemd160Gas         = big.NewInt(600)    //
    Ripemd160WordGas     = big.NewInt(120)    //
-   MinimumDifficulty    = big.NewInt(131072) // The minimum that the difficulty may ever be.
+   MinimumDifficulty    = big.NewInt(1)      // The minimum that the difficulty may ever be.
    CallCreateDepth      = big.NewInt(1024)   // Maximum depth of call/create stack.
    ExpGas               = big.NewInt(10)     // Once per EXP instuction.
    LogGas               = big.NewInt(375)    // Per LOG* operation.

Just to elaborate on Dennis' answer and explain why this hindrance exists.

Deep down, the decentralized nature of the consensus means that no one's clock is any better than anyone else'. Blocks disambiguate transaction order which is critical for reaching consensus about world state. They do this without reference to a reliable time source. Since no one can check the timeStamp without reference to a non-deterministic time source, the accuracy of the time stamp is dubious.

There's an interesting discussion on the accuracy of the timeStamp, the latitude the miners have and incentives to keep the timeStamp within reason, here: How do Ethereum mining nodes maintain a time consistent with the network?

This is quite a departure from networks synced with atomic clocks as we've come to take for granted, I think. A good understanding can keep us away from hidden assumptions and problems.

We can say ...

"The event will unfold at block _____" which is probably going to occur at approximately date/time given a predictable average block time.

"The event will unfold when a block is mined with a timeStamp >= deadline" which should be roughly at said deadline but isn't guaranteed to be precise.

"The event will unfold when the Oracle injects a transaction to declare that the event has occured without regard to either the block number or the block time stamp" in which case a trusted time source will decide.

Above are some examples of plain english we might use to describe the way a future time is codified in a Smart Contract so things unfold "as advertised."

Hope it helps.