code: func (b *BlockGen) OffsetTime(seconds int64) {
  	b.header.Time += uint64(seconds)
  	if b.header.Time <= b.parent.Header().Time {
  		panic("block time out of range")
  	}
  	chainreader := &fakeChainReader{config: b.config}
  	b.header.Difficulty = b.engine.CalcDifficulty(chainreader, b.header.Time, b.parent.Header())
  }
docs: // OffsetTime modifies the time instance of a block, implicitly changing its
  // associated difficulty. It's useful to test scenarios where forking is not
  // tied to chain length directly.