Bitcoin needs to know the current time in order to validate blocks. How does it decide that?
Note that I'm talking about the Satoshi client specifically.
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2 Answers
It takes the median time of the other clients connected, but only
1. if there are at least 5, and
2. if the median time does not differ from system time by more than 70 minutes.
For specifics, we look at AddTimeData, in timedata.cpp.
Note: I have edited it down for length
void AddTimeData(const CNetAddr& ip, int64 nTime)
{
int64 nOffsetSample = nTime - GetTime();
// Add data
vTimeOffsets.input(nOffsetSample);
if (vTimeOffsets.size() >= 5 && vTimeOffsets.size() % 2 == 1)
{
int64 nMedian = vTimeOffsets.median();
// Only let other nodes change our time by so much
if (abs64(nMedian) < 70 * 60)
{
nTimeOffset = nMedian;
}
else
{
nTimeOffset = 0;
}
}
}
answered Feb 1, 2013 at 9:05
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1@abeikverdi A full answer to that question would be too large to be contained within a single comment. It's an interesting question, though, and I encourage you to ask it as a separate question. Related: Was a timejacking attack ever performed? Mar 8, 2015 at 6:28
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Yea, that's a big question! Thanks for the link! I guess I need to read more about timejacking in Bitcoin's network Mar 8, 2015 at 6:37
As individual clients may have an arbitrary timeshift, the Satoshi client will use the median of its neighbors times alongs with its own time to find an offset to the local clock. This offset will then be used throughout the client wherever an accurate time is needed.
