DateWithoutTimePropertyValue.cs - 

using Newtonsoft.Json;

namespace Notion.Client;

/// <summary>
/// Date without time property value object.
/// </summary>
public class DateWithoutTimePropertyValue : PropertyValue
{
    public override PropertyValueType Type => PropertyValueType.Date;

    /// <summary>
    /// Date
    /// </summary>
    [JsonProperty("date", NullValueHandling = NullValueHandling.Include)]
    public DateWithoutTime Date { get; set; }
}

/// <summary>
/// Date without time value object.
/// </summary>
public class DateWithoutTime
{
    [JsonProperty("start")]
    private string? StartValue
    {
        get => Start?.ToString("yyyy-MM-dd");
        set => Start = value is null ? null : DateOnly.Parse(value);
    }

    /// <summary>
    /// Start date with optional time.
    /// </summary>
    [JsonIgnore]
    public DateOnly? Start { get; set; }

    [JsonProperty("end")]
    private string? EndValue
    {
        get => End?.ToString("yyyy-MM-dd");
        set => End = value is null ? null : DateOnly.Parse(value);
    }

    /// <summary>
    /// End date with optional time.
    /// </summary>
    [JsonIgnore]
    public DateOnly? End { get; set; }
}

await client.Pages.UpdatePropertiesAsync("00000000000000000000000000000000", new Dictionary<string, PropertyValue>()
{
    {
        "aaa", new DatePropertyValue
        {
            Id = "aaa",
            Date = new()
            {
                Start = DateOnly.FromDateTime(DateTime.UtcNow)
            }
        }
    },
    {
        "bbb", new DatePropertyValue
        {
            Id = "bbb",
            Date = new()
            {
                Start = DateOnly.FromDateTime(DateTime.UtcNow),
                End = DateOnly.FromDateTime(DateTime.UtcNow.AddDays(1))
            }
        }
    }
});