More implementation.

Modified intent of ts_db_records. timestamp value is no longer a
  {Date, Time} value, it is now gregorian seconds since year 0.
  This was done to make queries that are based on date comparisons
  easier to write and more efficient.
Added date time comparison function to ts_common.
Implemented update/1 and list/3 for ts_entry.

src/ts_entry.erl

@@ -17,3 +17,41 @@ new(ER = #ts_entry()) ->
         ok = mnesia:write(NewRow),
         NewRow end),
     {ok, NewRow}.
+
+update(ER = #ts_entry()) ->
+    
+    % look for existing record
+    case mnesia:dirty_read(ts_entry, ER#ts_entry.ref) of
+        % record does not exist
+        [] -> no_record;
+        % record exists, update it
+        [_Record] -> mnesia:dirty_write(ER)
+    end.
+
+list({Username, Timeline}, Start, Length)
+when is_integer(Start) and is_integer(Length) ->
+    ts_common:list(
+        qlc:q([E || E <- mnesia:table(ts_entry),
+                    E#ts_entry.ref =:= {Username, Timeline, _}]_,
+            Start, Length).
+
+list({Username, Timeline}, StartDateTime, EndDateTime) ->
+
+    {atomic, Entries} = mnesia:transaction(fun() ->
+
+        StartSeconds = calendar:datetime_to_gregorian_seconds(StartDateTime),
+        EndSeconds = calendar:datetime_to_gregorian_seconds(EndDateTime),
+
+        % create the query that will select these records
+        Q = qlc:q([E || E <- mnesia:table(ts_entry),
+                        E#ts_entry.timestamp >= StartSeconds,
+                        E#ts_entry.timestamp < EndSeconds]),
+
+        % sort by timestamp
+        SortedQ = qlc:sort(Q, {order, fun(A, B) ->
+            A#ts_entry.timestamp > B#ts_entry.timestamp end}),
+
+        % return
+        qlc:e(SortedQ)
+    end),
+    Entries.