Converting Strings For Mathematical Calculations?

I'm new to .NET and converting and proper way to convert strings to other values. Specifically, here is a situation I'm facing:

I'm returning two values from a datasource as strings. One values is a representation of a span of time, and another is a count of how many attempts to access a system took place within the span of time.

Given these parameters (for example):

Span of time: 00:01:00
Attempts: 13

I'd like to calculate a third value that shows how many seconds each attempt to connect took place during that span of time. This involves converting the time span to seconds and then dividing by attempts. Where I am getting lost is with the conversion of types. I know the span of time needs to be converted to a date, reduced to seconds, then converted to a number to be divided by the attempts field, also converted to a number.

I've been playing around with code segments like this all morning to no avail:

VBNET Code:
Dim thisTime As String = "00:01:00"
Dim thisAttempts As String = "13"
Label1.Text = thisTime
Label2.Text = thisAttempts
Label3.Text = Math.Round(Integer.Parse(Convert.ToDateTime(Label1.Text)) / Integer.Parse(Label2.Text), 2)