На самом деле XAxis.Min и XAxis.Max являются XLDate.Таким образом, ответ заключается в использовании XLDateToCalendarDate
Private Sub gcMain_ZoomEvent(ByVal sender As ZedGraphControl, ByVal oldState As ZoomState, ByVal newState As ZoomState) Handles gcMain.ZoomEvent
Dim xlMin, xlMax As Double
Dim year, month, day, hour, minute As Integer
Dim second As Double
Dim dateStart As Date
Dim dateEnd As Date
xlMin = gcMain.GraphPane.XAxis.Scale.Min
xlMax = gcMain.GraphPane.XAxis.Scale.Max
ZedGraph.XDate.XLDateToCalendarDate(xlMin, year, month, day, hour, minute, second)
dateStart = New Date(year, month, day, hour, minute, CInt(second))
ZedGraph.XDate.XLDateToCalendarDate(xlMax, year, month, day, hour, minute, second)
dateEnd = New Date(year, month, day, hour, minute, CInt(second))
SetDateText(textbox1.TextBox, dateStart)
SetDateText(text)
End Sub
Private Sub SetDateText(ByRef tbx As TextBox, ByVal dte As Date)
Try
tbx.Text = dte.Hour.ToString("00") + " : " + dte.Minute.ToString("00")
Catch
End Try
End Sub