[rrd-users] CDEF:a=TIME throws error

[itsmemattchung]

Hey Everyone,
I am new to the forum and I hope that after I learn and become more
experienced with rrdtool and its capabilities, I too can soon contribute to
this amazing tool.

I have followed the documentation
http://oss.oetiker.ch/rrdtool/tut/cdeftutorial.en.html
in order to calculate the derivative by using 
CDEF:a=TIME
CDEF:b=PREV(a)

"rrdtool.error: rpn expressions without DEF or CDEF variables are not
supported "

After searching around on google and through existing tickets, I ran across
http://oss.oetiker.ch/rrdtool-trac/ticket/182 which states exactly what I am
experiencing. 

Unfortunately, I am having a hard time interpreting what oetiker posted
"I agree, but ugliness is not a bug :-) even the error message you get is
correct. The reason for this is that CDEF builds a new time series which
does sort of not work if none of the parameters is a time series. Have a
look at the code and tell me how rrdgraph should handle this."

When would the syntax from the tutorial (CDEF:a=TIME) be appropriate? If
possible, can someone please elaborate on why that doesn't work, or point be
in the right direction so I can better understand?
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