diff --git a/src/Duration.zig b/src/Duration.zig
index 821e49a..e16e521 100644
--- a/src/Duration.zig
+++ b/src/Duration.zig
@@ -177,14 +177,14 @@ pub fn toString(self: Duration, allocator: std.mem.Allocator, options: ToStringR
     var write = abi.DiplomatWrite.init(allocator);
     defer write.deinit();
 
-    const res = abi.c.temporal_rs_Duration_to_string(self._inner, options, &write.inner);
+    const res = abi.c.temporal_rs_Duration_to_string(self._inner, options.toCApi(), &write.inner);
     try handleVoidResult(res);
 
     return try write.toOwnedSlice();
 }
 
 pub fn toJSON(self: Duration, allocator: std.mem.Allocator) ![]u8 {
-    return self.toString(allocator, defaultToStringRoundingOptions());
+    return self.toString(allocator, .{});
 }
 
 pub fn toLocaleString(self: Duration, allocator: std.mem.Allocator) ![]u8 {
@@ -214,26 +214,8 @@ fn wrapDuration(res: anytype) !Duration {
     return .{ ._inner = ptr };
 }
 
-fn defaultToStringRoundingOptions() ToStringRoundingOptions {
-    return abi.to_string_rounding_options_auto;
-}
-
 // --- Public helper types -----------------------------------------------------
 
-/// Options for Duration.toString()
-/// See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal/Duration/toString
-pub const ToStringOptions = struct {
-    /// Specifies the number of fractional seconds digits to display.
-    fractional_second_digits: ?u8 = null,
-
-    /// Specifies how to round off fractional second digits.
-    /// Defaults to "trunc" (truncate).
-    rounding_mode: ?RoundingMode = null,
-
-    /// Specifies the smallest unit to include in the output.
-    smallest_unit: ?Unit = null,
-};
-
 pub const PartialDuration = abi.c.PartialDuration;
 
 /// Relative-to context for duration operations.
@@ -253,7 +235,8 @@ const Unit_option = abi.c.Unit_option;
 const RoundingMode_option = abi.c.RoundingMode_option;
 
 pub const RoundingOptions = temporal.RoundingOptions;
-pub const ToStringRoundingOptions = abi.c.ToStringRoundingOptions;
+pub const ToStringOptions = temporal.ToStringRoundingOptions;
+pub const ToStringRoundingOptions = temporal.ToStringRoundingOptions;
 pub const Unit = temporal.Unit;
 pub const RoundingMode = temporal.RoundingMode;
 pub const Sign = temporal.Sign;
@@ -361,7 +344,7 @@ test toString {
     defer dur.deinit();
 
     const allocator = std.testing.allocator;
-    const str = try dur.toString(allocator, defaultToStringRoundingOptions());
+    const str = try dur.toString(allocator, .{});
     defer allocator.free(str);
 
     // Should output ISO 8601 duration format
diff --git a/src/Instant.zig b/src/Instant.zig
index 3d07582..b58b31e 100644
--- a/src/Instant.zig
+++ b/src/Instant.zig
@@ -362,7 +362,7 @@ test until {
         .largest_unit = .hour,
         .smallest_unit = .second,
         .rounding_mode = .trunc,
-        .increment = null,
+        .rounding_increment = null,
     };
 
     var until_handle = try earlier.until(later, settings);
@@ -386,7 +386,7 @@ test since {
         .largest_unit = .hour,
         .smallest_unit = .second,
         .rounding_mode = .trunc,
-        .increment = null,
+        .rounding_increment = null,
     };
 
     var until_handle = try earlier.until(later, settings);
@@ -408,7 +408,7 @@ test round {
         .largest_unit = null,
         .smallest_unit = .second,
         .rounding_mode = .half_expand,
-        .increment = null,
+        .rounding_increment = null,
     };
 
     const rounded = try inst.round(opts);
diff --git a/src/temporal.zig b/src/temporal.zig
index 38506b4..3761a80 100644
--- a/src/temporal.zig
+++ b/src/temporal.zig
@@ -95,35 +95,66 @@ pub const Sign = enum {
 };
 
 /// Options for rounding operations (Instant.round, Duration.round, etc.).
+/// MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal/Duration/round
 pub const RoundingOptions = struct {
     largest_unit: ?Unit = null,
     smallest_unit: ?Unit = null,
     rounding_mode: ?RoundingMode = null,
-    increment: ?u32 = null,
+    rounding_increment: ?u32 = null,
 
     pub fn toCApi(self: RoundingOptions) c.RoundingOptions {
         return .{
             .largest_unit = abi.toUnitOption(toCUnit(self.largest_unit)),
             .smallest_unit = abi.toUnitOption(toCUnit(self.smallest_unit)),
             .rounding_mode = abi.toRoundingModeOption(toCRoundingMode(self.rounding_mode)),
-            .increment = abi.toOption(c.OptionU32, self.increment),
+            .increment = abi.toOption(c.OptionU32, self.rounding_increment),
         };
     }
 };
 
 /// Options for computing differences between instants.
+/// MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal/Instant/until
 pub const DifferenceSettings = struct {
     largest_unit: ?Unit = null,
     smallest_unit: ?Unit = null,
     rounding_mode: ?RoundingMode = null,
-    increment: ?u32 = null,
+    rounding_increment: ?u32 = null,
 
     pub fn toCApi(self: DifferenceSettings) c.DifferenceSettings {
         return .{
             .largest_unit = abi.toUnitOption(toCUnit(self.largest_unit)),
             .smallest_unit = abi.toUnitOption(toCUnit(self.smallest_unit)),
             .rounding_mode = abi.toRoundingModeOption(toCRoundingMode(self.rounding_mode)),
-            .increment = abi.toOption(c.OptionU32, self.increment),
+            .increment = abi.toOption(c.OptionU32, self.rounding_increment),
+        };
+    }
+};
+
+/// Options for Duration.toString() formatting.
+/// MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Temporal/Duration/toString
+pub const ToStringRoundingOptions = struct {
+    fractional_second_digits: ?u8 = null,
+    smallest_unit: ?Unit = null,
+    rounding_mode: ?RoundingMode = null,
+
+    pub fn toCApi(self: ToStringRoundingOptions) c.ToStringRoundingOptions {
+        const precision: c.Precision = if (self.fractional_second_digits) |fsd|
+            .{ .is_minute = false, .precision = abi.toOption(c.OptionU8, fsd) }
+        else if (self.smallest_unit) |su|
+            switch (su) {
+                .second => .{ .is_minute = false, .precision = abi.toOption(c.OptionU8, 0) },
+                .millisecond => .{ .is_minute = false, .precision = abi.toOption(c.OptionU8, 3) },
+                .microsecond => .{ .is_minute = false, .precision = abi.toOption(c.OptionU8, 6) },
+                .nanosecond => .{ .is_minute = false, .precision = abi.toOption(c.OptionU8, 9) },
+                else => .{ .is_minute = false, .precision = abi.toOption(c.OptionU8, null) },
+            }
+        else
+            .{ .is_minute = false, .precision = abi.toOption(c.OptionU8, null) };
+
+        return .{
+            .precision = precision,
+            .smallest_unit = abi.toUnitOption(toCUnit(self.smallest_unit)),
+            .rounding_mode = abi.toRoundingModeOption(toCRoundingMode(self.rounding_mode)),
         };
     }
 };