diff --git a/core/headerchain.go b/core/headerchain.go
index 78ab09cd95..d4522e709b 100644
--- a/core/headerchain.go
+++ b/core/headerchain.go
@@ -528,3 +528,10 @@ func (hc *HeaderChain) Engine() consensus.Engine { return hc.engine }
 func (hc *HeaderChain) GetBlock(hash common.Hash, number uint64) *types.Block {
 	return nil
 }
+
+// BigIntSlice attaches the methods of sort.Interface to []*big.Int, sorting in increasing order. (used by CalcPastMedianTime)
+type BigIntSlice []*big.Int
+
+func (s BigIntSlice) Len() int           { return len(s) }
+func (s BigIntSlice) Less(i, j int) bool { return s[i].Cmp(s[j]) < 0 }
+func (s BigIntSlice) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }