Difficulty RFC: Clarify some ambiguities in the Zcash specification

book/src/dev/rfcs/0006-contextual-difficulty.md

@@ -114,11 +114,13 @@ that there will be at least 28 blocks in any relevant chain on mainnet and
 testnet.
 
 Difficulty threshold calculations are performed using unsigned 256-bit integers.
-Time calculations are performed using unsigned 32-bit integers.
+In the Zcash specification, time values are 32-bit integers. But the difficulty
+adjustment calculations include time subtractions which could overflow an
+unsigned type, so they are performed using signed 64-bit integers in `zcashd`.
 
-TODO:
-  - check how zcashd implements signed median time differences
-  - open a ticket to update the Zcash spec
+Zebra is free to implement its calculations in any way that produces equivalent
+results. Using `u256` difficulty and `i64` times will allow us to write simple
+Rust code which produces the correct results.
 
 ## State service interface changes
 [state-service-interface]: #state-service-interface
@@ -177,10 +179,13 @@ chain.
 Zcash uses block difficulty thresholds in its difficulty adjustment calculations.
 (Block hashes are not used for difficulty adjustment.)
 
-TODO:
-  - check if zcashd truncates the MeanTarget before dividing by
-    AveragingWindowTimespan (as well as after the division), and
-  - open a ticket to update the Zcash spec
+Note that `zcashd` truncates the `MeanTarget` after the mean calculation, and
+after dividing by `AveragingWindowTimespan`, but only one of these truncations
+is reflected in the Zcash specification.
+
+See [ZIP PR 418] for details.
+
+[ZIP PR 418]: https://github.com/zcash/zips/pull/418
 
 #### Median timespan
 [median-timespan]: #median-timespan
@@ -197,8 +202,6 @@ Calculated using the difference of the median timespans for:
 but this terminology is confusing, because it is a difference of medians, rather
 than any "actual" elapsed time.)
 
-TODO: open a Zcash spec clarification ticket
-
 The median timespan is damped by the `PoWDampingFactor`, and bounded by
 `PoWMaxAdjustDown` and `PoWMaxAdjustUp`.