How to add timestamp before update

Apologies if I state the obvious -- I think it's really better if in general people understand the code before modifying it. (maybe you already did this part? I wasn't sure from your post what you had considered)

The "LEFT" expression is supposed to display a specific number of characters from the left side of the string, and the syntax is LEFT(string, length)

The "LEN" expression is supposed to give you the length of a string and the syntax is LEN(string)

With this in mind, you should be striving to understand the original calculation, i.e. how does it get to the output of "<update> <date> | <previous update history>".

Step 1: reduce your calculation to the start and finish in order to match the "left" expression's syntax.

LEFT(LEAVE THIS PART OUT,2000)

Whatever it is you're doing, has a limit of 2000 characters.

Step 2: Breaking down the string to understand it better. 

IF(LEFT({DE:Instructions Edit History},LEN(IF(ISBLANK({DE:Instructions}),"-",{DE:Instructions})))={DE:Instructions},{DE:Instructions Edit History},CONCAT(IF(ISBLANK({DE:Instructions}),"-",{DE:Instructions})," (",$$NOW,") | ",{DE:Instructions Edit History}))

Step 2.A) The whole string is an IF statement -- you are looking for the following syntax: 

IF(condition, trueExpression, falseExpression)

 Broken down, your original statement looks like this:

IF(

HERE'S YOUR CONDITION

LEFT({DE:Instructions Edit History},LEN(IF(ISBLANK({DE:Instructions}),"-",{DE:Instructions})))={DE:Instructions},

HERE'S YOUR TRUE EXPRESSION

{DE:Instructions Edit History},

HERE'S YOUR FALSE EXPRESSION

CONCAT(IF(ISBLANK({DE:Instructions}),"-",{DE:Instructions})," (",$$NOW,") | ",{DE:Instructions Edit History}))

Please note your condition contains another IF statement, so you could choose to break this down in the same way.

Step 2.B) Understanding the specific condition:

Display this much of the calculated field: if the instructions field is blank, it's a dash, otherwise it's the length of whatever is in the instructions field 

 The contents of the field you're tracking.

In other words, the first part of the calculated field should exactly match whatever field it is that you're tracking.

Step 2.C) Understanding the true and false expressions.

If it's true, if they are both equal, just leave it alone -- it is what it is. 

If it's not true, tack the contents of the instructions field and a date ("now") in front of the existing calculated field.

With all that out of the way, I have a couple of initial observations which led me to write up this obvious-sounding response.

1) Your second IF statement -- inside your LEN expression -- appears to have some problems. If you consider the initial syntax I offered (if condition, trueExpression, falseExpression) -- you can see your IF statement is more like "if this is blank," followed by 4 different expressions, none of which are bracketed in any way. Work on what it is you're trying to say here, understanding that the eventual answer needs to be able to be expressed as a LEN.

2) You have included a third field, "GCX_Instructions" -- not sure if this is a typo or if the third field has some bearing on the rest of the calculation. Only you can say.

I hope this is of some small help.