@code {
  [Parameter]
  [EditorRequired]
  public SimpleTimeOnly Time { get; set; } = default!;

  [Parameter]
  [EditorRequired]
  public Action<SimpleTimeOnly> UpdateTime { get; set; }= default!;

  private string AmPm
  {
    get => Time.Hour < 12 ? "AM" : "PM";
  }
  private int AdjustedHour
  {
    get => convert24to12Hour(Time.Hour);
  }
  private int convert24to12Hour(int hour)
  {
    if (hour == 0)
    {
      // 12 AM
      return 12;
    }
    else if (hour <= 12)
    {
      // AM hours
      return hour;
    }
    else
    {
      // PM hours
      return hour - 12;
    }
  }
  private int convert12To24Hour(int hour, string? amPm)
  {
    if (amPm == null) return -1;
    if (amPm.ToUpper() == "PM" && hour < 12)
    {
      return hour + 12;
    }
    else if (amPm.ToUpper() == "AM" && hour == 12)
    {
      return 0;
    }
    else
    {
      return hour;
    }
  }
}
<div>
<select 
  @onchange="async (e) => 
    UpdateTime(
      new SimpleTimeOnly
      {
        Hour=convert24to12Hour(Convert.ToInt32(e.Value)),
        Minute=Time.Minute
      }
    )" 
  class="form-control w-auto d-inline"
>
  @foreach (var hour in Enumerable.Range(1, 12))
  {
    <option 
      value="@hour" 
      selected="@(hour == Time.Hour)"
    >
      @hour.ToString("00")
    </option>
  }
</select>
<span class="pl-0">:</span>
<select 
  @onchange="async (e) => 
    UpdateTime(
      new SimpleTimeOnly
      {
        Hour=AdjustedHour,
        Minute=Convert.ToInt32(e.Value)
      }
    )"
  class="form-control w-auto d-inline"
>
  @foreach (var minute in new int[] {0, 15, 30, 45, 59})
  {
    <option 
      value="@minute" 
      selected="@(minute == Time.Minute)"
    >
      @(minute.ToString("00"))
    </option>
  }
</select>

<select 
  @onchange="(e) => 
    UpdateTime(
      new SimpleTimeOnly
      {
        Hour=convert12To24Hour(Time.Hour, e.Value?.ToString()),
        Minute=Time.Minute
      }
    )"
  class="form-control w-auto d-inline"
>
  @foreach (var amPm in new string[] {"AM", "PM"})
  {
    <option 
      value="@amPm" 
      selected="@(amPm == AmPm)"
    >
      @amPm
    </option>
  }
</select>
</div>