Argthtjtr

Using two buttons to start and stop each using a

start_time = Format(Date.Now, "HH:mm:ss") 

or

stop_time = Format(Date.Now, "HH:mm:ss") 

to store the time and when submitting it uses a timespan to calculate the difference which works like a treat

Dim duration As TimeSpan = stop_time - start_time

Say if i recorded 00:59:00 minutes, how would i minus 00:20:00?, im guessing it would be another timespan with duration minus 00:20:00 but not sure how to get the 00:20:00 in a format to use this.

Cheers,
Pete

As of now, you're using strings to store DateTime values. There's no reason for it.

Strings should be used - when dealing with DateTime structure - for presentation only.

Use a TimeSpan to perform these calculations.

Store the start time of an operation:

Dim StartTime As TimeSpan = Date.Now.TimeOfDay

Store the end time of an operation:

Dim StopTime As TimeSpan = Date.Now.TimeOfDay

Calculate the Duration:

 Dim Duration As TimeSpan = StopTime - StartTime

A TimeSpan value can be created with:

[TimeSpan] = new TimeSpan([Hours], [Minutes], [Seconds]) or:
[TimeSpan] = TimeSpan.FromSeconds([Seconds]) | TimeSpan.FromMinutes([Minutes]) | .FromHours([Hours]) etc.

Note that the first form accepts Integer values, while the second form accepts Double values: when inserting the value in minutes, add a # to the value (indicating that the value is a Double type) and see what happens.

Create a new TimeSpan of 20 minutes and subtract it from the Duration value:

Dim SubSpan As TimeSpan = New TimeSpan(0, 20, 0)

'Or

Dim SubSpan As TimeSpan = TimeSpan.FromMinutes(20.0#)



Duration -= SubSpan

Use the TimeSpan.ToString() method to present the DateTime / TimeSpan value in a human readable, localized, form:

TextBox1.Text = Duration.ToString()

Or using a specific format:

TextBox1.Text = duration.ToString("hh:mm:ss")

TextBox1.Text = duration.ToString("hh:mm:ss", CultureInfo.InvariantCulture)

TextBox1.Text = duration.ToString("hh:mm:ss", CultureInfo.CreateSpecificCulture("en-UK"))

As noted in the comments (and in many other questions every day), it would be
a great idea to set Option Strict On, possibly Option Explicit On
and (it can be really useful) Option Infer On in the Project properties.
This will help a lot in making the right decision when declaring and using your Types.