[Copilot]

DateOnly and TimeOnly (introduced in .NET 6) had no polyfill, so they were unavailable on older target frameworks.

Type polyfills

• Added PolyShim/Net60/DateOnly.cs and PolyShim/Net60/TimeOnly.cs, guarded to compile only where the native types are unavailable (older .NET Core, .NET Framework, .NET Standard).
• Backed by DateTime/ticks internally; cover construction, arithmetic (AddDays/AddMonths/AddYears, Add/AddHours/AddMinutes, IsBetween), comparison/equality, Deconstruct, ToString/Parse/TryParse/ParseExact/TryParseExact (string and ReadOnlySpan<char> overloads), TryFormat, and conversions to/from DateTime/TimeSpan.

Tests

• Added PolyShim.Tests/Net60/DateOnlyTests.cs and TimeOnlyTests.cs covering the new API surface.

Docs

• Regenerated Signatures.md.
• Listed DateOnly/TimeOnly in Readme.md's type polyfills section.

using System;

// Works uniformly across all target frameworks
var date = new DateOnly(2023, 5, 17);
var time = new TimeOnly(13, 45, 30);
var dateTime = date.ToDateTime(time);

[Tyrrrz]

@copilot while the types have been introduced in .NET 6, have you checked whether all members you've polyfilled were also introduced in .NET 6 or maybe some of them were introduced later?

[Copilot]

Pull request overview

Adds polyfills for the .NET 6 DateOnly and TimeOnly types to PolyShim so consumers can use these APIs on older target frameworks, along with accompanying tests and documentation updates.

Changes:

• Added System.DateOnly and System.TimeOnly polyfill implementations under PolyShim/Net60/.
• Added Net60 test coverage for the new types.
• Updated README and regenerated Signatures.md to include the new polyfilled types.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
Signatures.md	Updates generated signature/type counts and lists DateOnly/TimeOnly.
Readme.md	Documents DateOnly/TimeOnly as available type polyfills.
PolyShim/Net60/DateOnly.cs	Introduces DateOnly polyfill implementation.
PolyShim/Net60/TimeOnly.cs	Introduces TimeOnly polyfill implementation.
PolyShim.Tests/Net60/DateOnlyTests.cs	Adds tests for DateOnly behavior.
PolyShim.Tests/Net60/TimeOnlyTests.cs	Adds tests for TimeOnly behavior.

Suppressed comments (12)

PolyShim/Net60/TimeOnly.cs:259

• TimeOnly.TryParse currently accepts strings that include a date component because it relies on DateTime.TryParse and then drops the date. Add a validation that the parsed date is the default (0001-01-01) produced by NoCurrentDateDefault, otherwise return false.

        if (
            s is not null
            && DateTime.TryParse(
                s,
                provider,

PolyShim/Net60/TimeOnly.cs:212

• TimeOnly.ParseExact(string, string, ...) uses DateTime.ParseExact and then drops the date, which can allow date components to sneak in via a format that includes them. Implement ParseExact in terms of TryParseExact so the same date-component validation is applied consistently.

    public static TimeOnly ParseExact(
        string s,
        string format,
        IFormatProvider? provider,
        DateTimeStyles style = DateTimeStyles.None

PolyShim/Net60/TimeOnly.cs:225

• TimeOnly.ParseExact(string, string[], ...) uses DateTime.ParseExact and then drops the date, which can allow date components to sneak in via a format that includes them. Implement ParseExact in terms of TryParseExact so the same date-component validation is applied consistently.

    public static TimeOnly ParseExact(
        string s,
        string[] formats,
        IFormatProvider? provider,
        DateTimeStyles style = DateTimeStyles.None

PolyShim/Net60/TimeOnly.cs:301

• TimeOnly.TryParseExact should reject inputs that include a date component. Add a check that the parsed date is 0001-01-01 (the default date produced by NoCurrentDateDefault when only a time is present).

        if (
            s is not null
            && format is not null
            && DateTime.TryParseExact(
                s,

PolyShim/Net60/TimeOnly.cs:336

• TimeOnly.TryParseExact (formats overload) should reject inputs that include a date component. Add a check that the parsed date is 0001-01-01 (the default date produced by NoCurrentDateDefault when only a time is present).

        if (
            s is not null
            && formats is not null
            && DateTime.TryParseExact(
                s,
                formats,

PolyShim/Net60/DateOnly.cs:145

• DateOnly.ParseExact uses DateTime.ParseExact and then drops the time. That allows date+time formats/inputs to parse successfully and silently discard the time. Implement ParseExact in terms of TryParseExact so the same time-component validation is applied consistently.

    public static DateOnly ParseExact(
        string s,
        string format,
        IFormatProvider? provider,
        DateTimeStyles style = DateTimeStyles.None

PolyShim/Net60/DateOnly.cs:155

• DateOnly.ParseExact (formats overload) uses DateTime.ParseExact and then drops the time. That allows date+time formats/inputs to parse successfully and silently discard the time. Implement ParseExact in terms of TryParseExact so the same time-component validation is applied consistently.

    public static DateOnly ParseExact(
        string s,
        string[] formats,
        IFormatProvider? provider,
        DateTimeStyles style = DateTimeStyles.None

PolyShim/Net60/DateOnly.cs:163

• DateOnly.ParseExact(ReadOnlySpan, ...) currently parses via DateTime.ParseExact directly and bypasses any validation added to the string-based ParseExact/TryParseExact. Delegate span overloads to the string overloads to keep behavior consistent.

    public static DateOnly ParseExact(
        ReadOnlySpan<char> s,
        ReadOnlySpan<char> format,
        IFormatProvider? provider = null,
        DateTimeStyles style = DateTimeStyles.None
    ) =>

PolyShim/Net60/DateOnly.cs:178

• DateOnly.ParseExact(ReadOnlySpan, string[] ...) currently parses via DateTime.ParseExact directly and bypasses any validation added to the string-based ParseExact/TryParseExact. Delegate span overloads to the string overloads to keep behavior consistent.

    public static DateOnly ParseExact(
        ReadOnlySpan<char> s,
        string[] formats,
        IFormatProvider? provider = null,
        DateTimeStyles style = DateTimeStyles.None
    ) => FromDateTime(DateTime.ParseExact(new string(s.ToArray()), formats, provider, style));

PolyShim/Net60/DateOnly.cs:194

• DateOnly.TryParse should reject inputs that include a time component. Add a check that the parsed time-of-day is exactly 00:00:00.000 (otherwise return false).

        if (s is not null && DateTime.TryParse(s, provider, style, out var dateTime))
        {
            result = FromDateTime(dateTime);
            return true;
        }

PolyShim/Net60/DateOnly.cs:228

• DateOnly.TryParseExact should reject inputs/formats that include a time component. Add a check that the parsed time-of-day is exactly 00:00:00.000 (otherwise return false).

        if (
            s is not null
            && format is not null
            && DateTime.TryParseExact(s, format, provider, style, out var dateTime)
        )

PolyShim/Net60/DateOnly.cs:256

• DateOnly.TryParseExact (formats overload) should reject inputs/formats that include a time component. Add a check that the parsed time-of-day is exactly 00:00:00.000 (otherwise return false).

        if (
            s is not null
            && formats is not null
            && DateTime.TryParseExact(s, formats, provider, style, out var dateTime)
        )

---

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[Copilot]

@copilot while the types have been introduced in .NET 6, have you checked whether all members you've polyfilled were also introduced in .NET 6 or maybe some of them were introduced later?

Good catch — not all polyfilled members were from .NET 6. Here's what I found and fixed:

Deconstruct methods — introduced in .NET 8 (dotnet/runtime#79499, merged January 2023):

• DateOnly.Deconstruct(out int year, out int month, out int day)
• TimeOnly.Deconstruct(out int hour, out int minute) (and the 3- and 4-param overloads)

These were previously placed in Net60/ (incorrect). I've moved them to Net80/ as extension methods using the standard PolyShim pattern, with a !NET8_0_OR_GREATER guard. The type polyfills in Net60/ no longer include them.

TimeOnly.ToDateTime() (no parameters) — not a public .NET API at all: This method is internal in the .NET runtime and was never part of the public surface. I changed it from public to private (it's only used internally by the ToString overloads).

All other polyfilled members (IsBetween, AddHours, AddMinutes, Add, FromDateTime, FromTimeSpan, ToTimeSpan, Parse/TryParse/ParseExact/TryParseExact, TryFormat, constructors, properties, operators) were correctly introduced in .NET 6.

The Signatures.md was regenerated automatically and now correctly marks Deconstruct entries as .NET 8.0. Commit: 78e24a5

[Tyrrrz]

@copilot please see other review comments and also fix build errors

[Copilot]

@copilot please see other review comments and also fix build errors

Fixed in commit Fix build errors and address review comments in DateOnly/TimeOnly polyfills:

• Build error: Added the missing private readonly DateTime _dateTime field declaration to DateOnly struct.
• DateOnly parsing: TryParse/TryParseExact now reject inputs with a non-zero time component; Parse/ParseExact delegate to the Try variants and throw FormatException on failure.
• TimeOnly parsing: TryParse/TryParseExact now reject inputs where the parsed date differs from DateTime.MinValue.Date (i.e., an explicit date was present); Parse/ParseExact delegate to the Try variants and throw FormatException on failure.
• TimeOnly.ToDateTime(DateOnly): Not added — the native BCL System.TimeOnly (verified on .NET 10) exposes no ToDateTime methods. The correct BCL API for combining is DateOnly.ToDateTime(TimeOnly), which is already in the polyfill. The parameterless ToDateTime() helper remains private.
• Tests: Added TryParse_WithTimeComponent_Failure_Test to DateOnlyTests and TryParse_WithDateComponent_Failure_Test to TimeOnlyTests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.24%. Comparing base (0d900be) to head (9ab8750).

Additional details and impacted files

@@            Coverage Diff             @@
##            prime     #138      +/-   ##
==========================================
+ Coverage   99.20%   99.24%   +0.03%     
==========================================
  Files         154      158       +4     
  Lines        4170     4383     +213     
  Branches       89       89              
==========================================
+ Hits         4137     4350     +213     
  Misses         13       13              
  Partials       20       20              

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[Tyrrrz]

Also apply the same suggestions to other test classes (not just DateOnly), where it's applicable @copilot

[Copilot]

Also apply the same suggestions to other test classes (not just DateOnly), where it's applicable @copilot

Done — applied all the same consolidations to TimeOnlyTests: Constants_Test at top, Add_Test merges Add/Add_WrapsAroundMidnight/AddHours/AddMinutes, IsBetween_Test merges the midnight-wrapping case, From_Test merges FromDateTime/FromTimeSpan, and Equals_Test absorbs Operators_Test and SubtractionOperator_WrapsAroundMidnight_Test.

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated no new comments.

Suppressed comments (1)

PolyShim/Net60/TimeOnly.cs:114

• FromTimeSpan(TimeSpan) currently accepts values outside the valid time-of-day range (e.g. >= 24h). That can produce TimeOnly instances whose Ticks/ToTimeSpan() represent more than a day, which is inconsistent with TimeOnly semantics (and differs from the BCL API, which throws for out-of-range inputs).

    public static TimeOnly FromDateTime(DateTime dateTime) =>
        new(new DateTime(dateTime.TimeOfDay.Ticks));

    public static TimeOnly FromTimeSpan(TimeSpan timeSpan) => new(new DateTime(timeSpan.Ticks));