Private Sub tmrAlarm_Timer()
Static next_alarm As Date
Dim secs_to_go As Single

    ' See if we have initialized next_alarm.
    If next_alarm = 0 Then
        ' Initialize next_alarm.
        next_alarm = NextAlarmTime
        lblNextAlarm.Caption = Format$(next_alarm, _
            "hh:mm:ss")
    End If

    ' See how many milliseconds until the next
    ' alarm time.
    secs_to_go = DateDiff("s", Now, next_alarm)

    ' Stop the timer to clear previous alarms.
    tmrAlarm.Enabled = False
    
    ' Set the timer's Interval.
    If secs_to_go < 1 Then
        ' Fire the alarm.
        ' *** Do something here ***
        Beep
        
        ' Reinitialize next_alarm.
        next_alarm = NextAlarmTime
    ElseIf secs_to_go < 60 Then
        tmrAlarm.Interval = secs_to_go * 1000
    Else
        tmrAlarm.Interval = 60000
    End If

    ' Restart the timer.
    tmrAlarm.Enabled = True
End Sub

' Return the next time that is a multiple of 5 minutes.
Private Function NextAlarmTime() As Date
' The number of minutes between alarms.
Const ALARM_INTERVAL = 5

Dim time_now As Date
Dim h As Integer
Dim m As Integer
Dim seconds As Long
Dim next_m As Integer

    ' Get the current time.
    time_now = Now

    ' Get the hour and minutes.
    h = DatePart("h", time_now)
    m = DatePart("n", time_now)

    ' Calculate the next multiple of ALARM_INTERVAL.
    next_m = ((m \ ALARM_INTERVAL) + 1) * ALARM_INTERVAL

    NextAlarmTime = DateAdd("n", h * 60 + next_m, Date)
End Function