2D rotation interpolation

I'm trying to figure out a nice way to interpolate rotation without it flipping. I have a working code for it, but just have this nudging feeling that there is a better way to do it.

Here is what I have now.

	Method AngleBetween:Float(  x1:Float, y1:Float, x2:Float, y2:Float  )
		Local xd:Float = (x1-x2)
		Local yd:Float = (y1-y2)
		Local m:Float = 90
		If xd < 0 Then m = 270
		Local Angle:Float = ATan(yd/xd) + m
		If Angle = "nan" Then Return 0 Else Return Angle
	End Method
	
 ' And elsewhere in the code

 	Method Update( Rotation:Int = 0)
		If Rotation And (Abs(Goal-Value)) > 180 Then
			If Value > 360 Then Value:- 360
			If Value < 0 Then Value:+ 360
			If Goal > 180 Then Goal:- 360 Else Goal:+ 360
		End If
		Local Distance:Float = Goal - Value
		Local Acceleration:Float = (Distance - Vel*Lag)*2/(Lag*Lag)
		Vel:+ Acceleration
		Value:+ Vel
	End Method

The Rotation input variable on the second method is just a flag to tell it if rotation is being interpolated, so that it can do some stuff to avoid flipping. The Update method take any number and moves it towards a goal with a physics like lag.

Is there a cleaner way to do this? The "If Angle = "nan" feels a bit hacky, and the whole thing that flips the rotation with lots of If's in the second Method too.

Cheers,

Ragnar