diff --git a/src/commands.zig b/src/commands.zig
index 7907b13..dfa6e6d 100644
--- a/src/commands.zig
+++ b/src/commands.zig
@@ -1977,6 +1977,7 @@ fn cmd_update(ctx: *Context, msg: *wire.Message, reply: *wire.Reply) !void {
                 "update.updates.arrayFilters",
             ) orelse return,
             .query = q,
+            .now_ms = std.Io.Timestamp.now(ctx.io, .real).toMilliseconds(),
             .diag = &diag,
         };
         // Before the scan, not after: an update naming an identifier nothing
@@ -2102,6 +2103,7 @@ fn cmd_find_and_modify(ctx: *Context, msg: *wire.Message, reply: *wire.Reply) !v
             "findAndModify.arrayFilters",
         ) orelse return,
         .query = q,
+        .now_ms = std.Io.Timestamp.now(ctx.io, .real).toMilliseconds(),
         .diag = &diag,
     };
     if (doc_arg(msg.body.get("update"))) |u| {
@@ -4313,6 +4315,17 @@ fn update_refusal(reply: *wire.Reply, err: anyerror, diag: update.Diagnostic) !v
             "Unrecognized or invalid $push modifier '{s}' at field '{s}'",
             .{ diag.segment, diag.path },
         )),
+        error.BadCurrentDateType => return bad_value(
+            reply,
+            "The '$type' string field is required to be 'date' or 'timestamp': " ++
+                "{$currentDate: {field : {$type: 'date'}}}",
+        ),
+        error.BadCurrentDateOperand => return bad_value(reply, try std.fmt.allocPrint(
+            arena,
+            "{s} is not valid type for $currentDate. Please use a boolean ('true') or a " ++
+                "$type expression ({{$type: 'timestamp/date'}}).",
+            .{diag.other},
+        )),
         error.PathNotViable => return reply.put_error(
             @intFromEnum(ErrorCode.path_not_viable),
             "PathNotViable",
@@ -4410,7 +4423,11 @@ fn build_upsert_doc(
     const owned = try arena.create(bson.Document);
     owned.* = bson.Document{ .arena = std.heap.ArenaAllocator.init(arena), .pairs = try pairs.toOwnedSlice(arena) };
     // Apply update operators to build the final doc; _id handled by insert.
-    try update.apply(owned, &.{ .arena = undefined, .pairs = u_doc }, opts);
+    // `inserting` is what `$setOnInsert` asks about, and this is the only
+    // caller that answers yes.
+    var insert_opts = opts;
+    insert_opts.inserting = true;
+    try update.apply(owned, &.{ .arena = undefined, .pairs = u_doc }, insert_opts);
     return owned;
 }
 
diff --git a/src/update.zig b/src/update.zig
index 6fa5d02..a6d804f 100644
--- a/src/update.zig
+++ b/src/update.zig
@@ -34,6 +34,10 @@ pub const UpdateError = error{
     PullAllNeedsArray,
     /// `$each` that is not an array.
     BadEach,
+    /// `$currentDate` handed a document that is not `{$type: date|timestamp}`.
+    BadCurrentDateType,
+    /// `$currentDate` handed something that is neither a bool nor a document.
+    BadCurrentDateOperand,
     /// A `$push` modifier that is unknown, or whose operand is not what it
     /// takes: `$slice`/`$position` want a number, `$sort` a number or a
     /// one-field document.
@@ -115,6 +119,15 @@ pub const Options = struct {
     /// The command's query. `$` resolves against it, so an update carrying one
     /// without this is refused rather than guessing at an element.
     query: ?[]const bson.Pair = null,
+    /// Whether this application is building a document an upsert will insert.
+    /// `$setOnInsert` is the one operator that asks.
+    inserting: bool = false,
+    /// The clock `$currentDate` reads, in Unix milliseconds. There is no
+    /// fallback to a global one: this file has no `io` to read the real clock
+    /// through, and a caller that forgets gets the epoch rather than a value
+    /// that changes between runs. The command handlers pass the same clock
+    /// `ttl_sweep` and the cursor sweep already use.
+    now_ms: i64 = 0,
     diag: ?*Diagnostic = null,
 };
 
@@ -636,6 +649,15 @@ fn apply_operator(
     // shape is checked once here rather than at the top of each.
     const ops = doc_pairs(value) orelse return error.InvalidUpdate;
     if (std.mem.eql(u8, op, "$set")) return op_set(arena, pairs, ops, opts);
+    // `$setOnInsert` is `$set` on the branch that inserts and nothing at all
+    // on the branch that updates -- including its `_id` rule, which is why it
+    // does not go through `op_set`: writing `_id` is allowed on a document
+    // being built and refused on one being rewritten. Measured.
+    if (std.mem.eql(u8, op, "$setOnInsert")) {
+        if (!opts.inserting) return;
+        return op_set_on_insert(arena, pairs, ops, opts);
+    }
+    if (std.mem.eql(u8, op, "$currentDate")) return op_current_date(arena, pairs, ops, opts);
     if (std.mem.eql(u8, op, "$unset")) return op_unset(arena, pairs, ops, opts);
     if (std.mem.eql(u8, op, "$inc")) return op_arith(arena, pairs, ops, opts, .add);
     if (std.mem.eql(u8, op, "$mul")) return op_arith(arena, pairs, ops, opts, .mul);
@@ -664,6 +686,72 @@ fn op_set(
     }
 }
 
+fn op_set_on_insert(
+    arena: std.mem.Allocator,
+    pairs: *std.ArrayListUnmanaged(bson.Pair),
+    ops: []const bson.Pair,
+    opts: Options,
+) UpdateError!void {
+    for (ops) |p| {
+        for (try resolve(arena, pairs.items, p.key, opts)) |segs| {
+            try set_path(arena, pairs, segs, try bson.copy_value(arena, p.value), p.key, opts.diag);
+        }
+    }
+}
+
+/// `$currentDate`: write the clock, as a date or as a BSON timestamp.
+///
+/// The operand says which. A bool -- **either** bool, measured: `false` writes
+/// a date too, the value is ignored -- means a date; `{$type: "date"}` and
+/// `{$type: "timestamp"}` say so; anything else is refused.
+fn op_current_date(
+    arena: std.mem.Allocator,
+    pairs: *std.ArrayListUnmanaged(bson.Pair),
+    ops: []const bson.Pair,
+    opts: Options,
+) UpdateError!void {
+    const now = opts.now_ms;
+    for (ops) |p| {
+        const value: bson.Value = switch (try current_date_kind(p.value, opts, p.key)) {
+            .date => .{ .datetime = now },
+            // A BSON timestamp is seconds in the high 32 bits and an ordinal
+            // in the low ones. mongod fills the ordinal from the oplog; a
+            // standalone has no oplog, so it is 1 -- the same answer twice in
+            // one second is the same timestamp, which nothing here reads.
+            .timestamp => .{ .timestamp = (@as(u64, @intCast(@divFloor(now, 1000))) << 32) | 1 },
+        };
+        for (try resolve(arena, pairs.items, p.key, opts)) |segs| {
+            try set_path(arena, pairs, segs, value, p.key, opts.diag);
+        }
+    }
+}
+
+fn current_date_kind(
+    v: bson.Value,
+    opts: Options,
+    path: []const u8,
+) UpdateError!enum { date, timestamp } {
+    switch (v) {
+        .bool => return .date,
+        .doc => |spec| {
+            const t = bson.get_pair(spec, "$type") orelse {
+                note(opts.diag, path, "");
+                return error.BadCurrentDateType;
+            };
+            if (t == .string) {
+                if (std.mem.eql(u8, t.string, "date")) return .date;
+                if (std.mem.eql(u8, t.string, "timestamp")) return .timestamp;
+            }
+            note(opts.diag, path, "");
+            return error.BadCurrentDateType;
+        },
+        else => {
+            if (opts.diag) |d| d.* = .{ .path = path, .other = v.type_name() };
+            return error.BadCurrentDateOperand;
+        },
+    }
+}
+
 fn op_unset(
     arena: std.mem.Allocator,
     pairs: *std.ArrayListUnmanaged(bson.Pair),
@@ -2424,3 +2512,85 @@ test "a $push modifier that is not one, or is handed the wrong thing, is refused
     }
     try testing.expectEqual(@as(usize, 1), doc.get("t").?.array.len);
 }
+
+test "$setOnInsert writes only on the branch that inserts" {
+    // Mutation check: drop the `opts.inserting` guard and the first half goes
+    // red -- an update would gain a field that is only supposed to exist on a
+    // document nobody had before.
+    var updating = try doc_with(testing.allocator, &.{
+        .{ .key = "_id", .value = .{ .int32 = 1 } },
+        .{ .key = "a", .value = .{ .int32 = 100 } },
+    });
+    defer updating.arena.deinit();
+    try apply(&updating, &doc_of(&.{
+        .{ .key = "$setOnInsert", .value = .{ .doc = &.{.{ .key = "a", .value = .{ .int32 = 1 } }} } },
+        .{ .key = "$set", .value = .{ .doc = &.{.{ .key = "b", .value = .{ .int32 = 2 } }} } },
+    }), .{});
+    try testing.expectEqual(@as(i32, 100), updating.get("a").?.int32);
+    try testing.expectEqual(@as(i32, 2), updating.get("b").?.int32);
+
+    var inserting = try doc_with(testing.allocator, &.{.{ .key = "k", .value = .{ .int32 = 1 } }});
+    defer inserting.arena.deinit();
+    try apply(&inserting, &doc_of(&.{
+        .{ .key = "$setOnInsert", .value = .{ .doc = &.{.{ .key = "a", .value = .{ .int32 = 1 } }} } },
+    }), .{ .inserting = true });
+    try testing.expectEqual(@as(i32, 1), inserting.get("a").?.int32);
+}
+
+test "$setOnInsert may write _id, where $set may not" {
+    // The one place the immutability rule does not apply: a document being
+    // built has no identity yet to change.
+    var doc = try doc_with(testing.allocator, &.{.{ .key = "k", .value = .{ .int32 = 1 } }});
+    defer doc.arena.deinit();
+    try apply(&doc, &doc_of(&.{
+        .{ .key = "$setOnInsert", .value = .{ .doc = &.{.{ .key = "_id", .value = .{ .int32 = 9 } }} } },
+    }), .{ .inserting = true });
+    try testing.expectEqual(@as(i32, 9), doc.get("_id").?.int32);
+
+    try testing.expectError(error.ImmutableId, apply(&doc, &doc_of(&.{
+        .{ .key = "$set", .value = .{ .doc = &.{.{ .key = "_id", .value = .{ .int32 = 8 } }} } },
+    }), .{ .inserting = true }));
+}
+
+test "$currentDate writes the clock it was handed" {
+    // The clock is a parameter, so the result is a value rather than a moving
+    // target -- and the server passes the same one `ttl_sweep` reads.
+    var doc = try doc_with(testing.allocator, &.{.{ .key = "a", .value = .{ .int32 = 1 } }});
+    defer doc.arena.deinit();
+    try apply(&doc, &doc_of(&.{
+        .{ .key = "$currentDate", .value = .{ .doc = &.{
+            .{ .key = "d", .value = .{ .bool = true } },
+            // Measured: `false` writes a date too. The boolean says "a date",
+            // not "whether".
+            .{ .key = "f", .value = .{ .bool = false } },
+            .{ .key = "e", .value = .{ .doc = &.{.{ .key = "$type", .value = .{ .string = "date" } }} } },
+            .{ .key = "t", .value = .{ .doc = &.{.{ .key = "$type", .value = .{ .string = "timestamp" } }} } },
+        } } },
+    }), .{ .now_ms = 1_700_000_000_123 });
+    try testing.expectEqual(@as(i64, 1_700_000_000_123), doc.get("d").?.datetime);
+    try testing.expectEqual(@as(i64, 1_700_000_000_123), doc.get("f").?.datetime);
+    try testing.expectEqual(@as(i64, 1_700_000_000_123), doc.get("e").?.datetime);
+    // Seconds in the high 32 bits, an ordinal in the low ones.
+    try testing.expectEqual(@as(u64, (1_700_000_000 << 32) | 1), doc.get("t").?.timestamp);
+}
+
+test "$currentDate refuses an operand that names no type" {
+    var doc = try doc_with(testing.allocator, &.{.{ .key = "a", .value = .{ .int32 = 1 } }});
+    defer doc.arena.deinit();
+    var diag: Diagnostic = .{};
+    try testing.expectError(error.BadCurrentDateType, apply(&doc, &doc_of(&.{
+        .{ .key = "$currentDate", .value = .{ .doc = &.{
+            .{ .key = "d", .value = .{ .doc = &.{.{ .key = "$type", .value = .{ .string = "nope" } }} } },
+        } } },
+    }), .{ .diag = &diag }));
+    try testing.expectError(error.BadCurrentDateType, apply(&doc, &doc_of(&.{
+        .{ .key = "$currentDate", .value = .{ .doc = &.{
+            .{ .key = "d", .value = .{ .doc = &.{.{ .key = "a", .value = .{ .int32 = 1 } }} } },
+        } } },
+    }), .{ .diag = &diag }));
+    try testing.expectError(error.BadCurrentDateOperand, apply(&doc, &doc_of(&.{
+        .{ .key = "$currentDate", .value = .{ .doc = &.{.{ .key = "d", .value = .{ .int32 = 1 } }} } },
+    }), .{ .diag = &diag }));
+    try testing.expectEqualStrings("int", diag.other);
+    try testing.expect(doc.get("d") == null);
+}