MVC Calendar pops up when checkbox clicked

The calendar control pops up when clicking the checkbox. This is a ASP.NET Zero, ASP.NET Boilerplate project. The latest date control selected will reappear when the checkbox is clicked. Any clue as to why the calendar pops up when the checkbox is clicked?

@Html.TextBoxFor(m => m.Date1, "{0:yyyy-MM-dd}", new { type = "date", @class = "form-control", id = "Date1"})
@Html.TextBoxFor(m => m.Date2, "{0:yyyy-MM-dd}", new { type = "date", @class = "form-control", id = "Date2" })
@Html.CheckBoxFor(m => m.ISChecked, new { @class = "icheck", id = "Checkbox1" })

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https://i.imgur.com/wMMPIWz.jpg