Holding a formula with variables inside another variable

Question

I decided I wanted to beautify my batch code by assigning a formula (which contains its own sub-variables ('hrs' and 'min' and 'sec') as well as numerical operations and math operators) to a variable ('myformula') at the top of my code, then call and expand the variable (%myformula%) later on in multiple "set /a ..." commands.

I have tried %%hrs%% and !hrs! and ^carets, as well as !myformula!, but I always get a "Missing operand." error. This suggests to me that the maths operators are not being expanded correctly, or that to numerical values are being converted to characters.

I should add that everything works perfectly if I admit defeat, and replace %myformula% in the code with the actual formula.

Screen output gives correct display of the formula -

My Formula : %hrs%*60*60*100+%min%*60*100+%sec%*100+%cen%
Missing operand. 
My Time : 
( OR - My Time : 1   depending on %, %%, !, ^, etc, etc ... ) 

Can anyone suggest how to make the formula expansion %myformula% work correctly?

(I would consider NON native solutions such as - use javascript, use cscript, use powershell - as well as defeatest answers like - "set /a mytime=the+original+formula" - as not being an answer to my real question).

@echooff 
SETLOCAL ENABLEDELAYEDEXPANSION

:start
set myformula=%hrs%*60*60*100+%min%*60*100+%sec%*100+%cen%

:timer
rem - Timer code credits go to - rberteig  (2014)
rem Convert t0 into a scaler in 100th of sec with no 
seperator chars. 
set /a t0=%time: =0%
set /a hrs=1%t0:~0,2%-100
set /a min=1%t0:~3,2%-100
set /a sec=1%t0:~6,2%-100
set /a cen=1%t0:~9,2%-100
echo My Formula : %myformula%
set /a mytime=%myformula%
rem - set /a mytime=!myformula!   - does not work either.
echo My Time : %mytime%

:finish
pause
ENDLOCAL
exit /B 0
:EOF

Answer 1

Ok, with lots more trial and error, I found the solution myself. When saving formula (which is intended for arithmetic evaluation) to a variable, no % character wrapping is required.

This is apparently because the SET coomand, when used with the /A switch, automatically assumes any alphabet characters to be variables, and expands them without requiring any % wrapping.

This is alluded to in the SET /? help topic, and is mentioned a bit more in various web pages and forums. The only example I could find in retrospect was -

https://ss64.com/nt/set.html

Thus the answer to my question is to use this code line when seting the "myformula" variable:

:start
set myformula=hrs*60*60*100+min*60*100+sec*100+cen

• which expands perfectly as intended at the later code line:

    set /a mytime=%myformula%
  

• as its preceding and proceeding echo lines prove from their screen output.

Who would have thought???