how to add 30 mins for current time - vb.net

Question

I am using VB.NET. I have a dropdownlist named startTimeDDL. Than I am using loop to enter in time inside that dropdownlist.
' Auto fill "Start Time" for DropDownList
Dim StartTime As DateTime = #12:00:00 AM#
For i As Integer = 0 To 47
StartTimeDDL.Items.Add(StartTime.ToString("hh:mm tt"))
StartTime = DateAdd(DateInterval.Minute, 30, StartTime)
Next
So look below and that will be inside the dropdownList. notie the format is hh:mm am/pm.
12:00 AM
12:30 AM
01:00 AM
01:30 AM
02:00 AM
...
11:30 PM
Problem:
lets say current time is 1:21:01 pm than I want to write code so it select 1:30 pm from dropdownlist. Now les take another example. les say current time is 12:00:00 AM than I was to select 12:30 AM from dropdownlist. les take one last example. les say current time is 2:10:12 AM than I want to select 2:30 AM from the dropdownlist.
here is the code I wrote so far. problem with it is that I am only selecting the current time. now can I modfily to do what I want?
Dim dDate As DateTime = DateTime.Now.ToString("hh:mm tt")
Dim temp As String = dDate
StartTimeDDL.Items.FindByValue(temp).Selected = True

Answer 1

Round up if the minute value exceeds 30, round down if it's lower.
Here is an example implementation, you'll need to decide what to do with the "exactly 30 minutes past the hour" edge case. In my code it'll round up for that too.
Private Function RoundDateToHalfHours() As Date
Dim current As DateTime = DateTime.Now
Dim ts As TimeSpan
If current.Minute >= 30 Then
ts = New TimeSpan(current.Hour + 1, 0, 0)
Else
ts = New TimeSpan(current.Hour, 30, 0)
End If
Return current.Date.AddTicks(ts.Ticks)
End Function
Usage:
Dim roundedDate As DateTime = RoundDateToHalfHours()
StartTimeDDL.Items.FindByValue(roundedDate.ToString("hh:mm tt")).Selected = True

Answer 2

You are playing fast and loose with DateTime when you should be using TimeSpan, some caution required. The general way to round up an integral number to an arbitrary interval is
roundedUp = interval * ((number + interval - 1) \ interval)
Which you can readily use on DateTime as well by converting it to ticks, a one-liner
Public Function RoundupDate(dt As DateTime, interval As TimeSpan) As DateTime
Return New DateTime(interval.Ticks * ((dt.Ticks + interval.Ticks - 1) \ interval.Ticks))
End Function
Sample usage:
Dim example = #2:10:12 AM#
Dim rounded = RoundupDate(example, TimeSpan.FromMinutes(30))

Answer 3

Wasn't sure what you meant by a 'dropdownList'. For this example I used a ComboBox.
Dim StartTime As DateTime = #12:00:00 AM#
'load combo box
Do
StartTimeDDL.Items.Add(StartTime.ToString("hh:mm tt"))
StartTime = StartTime.AddMinutes(30)
Loop While StartTime.TimeOfDay.TotalDays > 0
Dim selectTime As DateTime = #2:10:12 PM# 'TEST find this <<<<<<<<<<<<<<<<<<<
'round time to 30 minutes
Dim numSecs As Integer = (CInt(selectTime.TimeOfDay.TotalSeconds) \ 1800) * 1800
'the OP said 'les say current time is 12:00:00 AM than I was to select 12:30 AM"
'so....
numSecs += 1800 'round up 30 minutes ????????
'create 'find'
Dim ts As New TimeSpan(0, 0, numSecs)
Dim findDate As New DateTime(ts.Ticks)
StartTimeDDL.SelectedIndex = StartTimeDDL.FindStringExact(findDate.ToString("hh:mm tt"))