Substring Problem: Uncaught TypeError

Hello everybody, first i want to improve myself, my name is simon and i´m from germany, my english is not so good, so please be merciful to me ;)

So here is my Problem, i want to use Substring in Titanium (build 3.4) but i get this error:

[ERROR] :  TiExceptionHandler: (main) [294,294] ----- Titanium Javascript Runtime Error -----
[ERROR] :  TiExceptionHandler: (main) [0,294] - In global/functions.js:48,25
[ERROR] :  TiExceptionHandler: (main) [1,295] - Message: Uncaught TypeError: Object 1388973670 has no method 'substring'
[ERROR] :  TiExceptionHandler: (main) [1,296] - Source:     var lTime1 = mytime.substring(-1,1);
[ERROR] :  V8Exception: Exception occurred at global/functions.js:48: Uncaught TypeError: Object 1388973670 has no method 'substring'

this is my code (as you can see in errormessage):

var lTime1 = mytime.substring(-1,1);

i dont know what to do and the only questions like mine that i can find are older than 4 years an use the same method... so i have no idea what i can do ^^

Thanks to every answer =)