Add Seconds To Timer

Below is my code to display a seconds count down timer in a textbox.

What I want to be able to do is have another textbox where I can enter the number of seconds that the timer will count down.

I don't know how to take a value from the textbox and make it like
#12:00:15 AM#

How would I do this?

Sub warmTimer()
Dim InitialTime As Single
Dim FinalTime As Single
InitialTime = Time
FinalTime = InitialTime + #12:00:15 AM#
Do
txtTimer.Text = Format((FinalTime - Time), "s")
Loop Until (Time >= FinalTime) Or (Skip = 1)
txtTimer.Text = "Time complete"
End Sub