feat(s7): DateTime (S7 DATE_AND_TIME) scalar read+write via S7.Net.Types.DateTime

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.S7/S7Driver.cs

@@ -590,7 +590,7 @@ public sealed class S7Driver
     internal static int ScalarByteWidth(S7TagDefinition tag) => tag.DataType switch
     {
         S7DataType.Int64 or S7DataType.UInt64 or S7DataType.Float64 => 8,
-        S7DataType.DateTime => 8,
+        S7DataType.DateTime => 8, // 8 = S7 DATE_AND_TIME/DT BCD (S7.Net.Types.DateTime; the 8-byte DT, not 12-byte DTL).
         S7DataType.String => tag.StringLength + 2,
         _ => throw new InvalidOperationException(
             $"S7 ScalarByteWidth called for non-buffer type {tag.DataType} (tag '{tag.Name}')"),
@@ -623,8 +623,11 @@ public sealed class S7Driver
             // padding past curLen — it validates curLen against the block, so no extra guard is needed.
             S7DataType.String => global::S7.Net.Types.S7String.FromByteArray(block),
 
-            S7DataType.DateTime => throw new NotSupportedException(
-                "S7 DateTime scalar reads land in a follow-up PR"),
+            // S7 classic DATE_AND_TIME (DT): 8-byte BCD value (year/month/day/hour/min/sec + ms +
+            // day-of-week nibble). S7.Net's DateTime.FromByteArray reads the BCD fields and returns a
+            // System.DateTime; the DT range is 1990–2089 and round-trips to the millisecond. Boxed as
+            // System.DateTime explicitly (the cast pins the box type, consistent with the numeric arms).
+            S7DataType.DateTime => (object)global::S7.Net.Types.DateTime.FromByteArray(block),
 
             _ => throw new System.IO.InvalidDataException(
                 $"S7 scalar Read type-mismatch: tag '{tag.Name}' declared {tag.DataType} but address " +
@@ -673,7 +676,11 @@ public sealed class S7Driver
                 return global::S7.Net.Types.S7String.ToByteArray(Convert.ToString(value) ?? "", tag.StringLength);
 
             case S7DataType.DateTime:
-                throw new NotSupportedException("S7 DateTime scalar writes land in a follow-up PR");
+                // S7.Net's DateTime.ToByteArray builds the 8-byte DT BCD block. It validates the
+                // 1990–2089 DT range and throws ArgumentOutOfRangeException for an out-of-range year —
+                // we surface that (no silent clamp). Value coerced via Convert.ToDateTime (accepts a
+                // System.DateTime or a parseable timestamp string).
+                return global::S7.Net.Types.DateTime.ToByteArray(Convert.ToDateTime(value));
 
             default:
                 throw new InvalidOperationException(