first, your post says 1145 + 15 ==> 0200 ...

I would like to get this done in simpler way.

Second, the only way to do it in a simple way (w.r.t to you) is to use a module and let it do all the work for you .. you don't want to be adding the time in there yourself and dealing w/the gregorian calendar .. e.g. add 15 minutes to each of these: 200002282345, 200402282345, 200602282345 ; and also 200512312345

Tell the people above you that you can a) spend a lot of time reinventing the wheel, then more time testing your code, and probably still have a bug or 2, or b) spend 5 min looking at modules docs and write a 2-line, very robust, very maintainable snippet.
Do we even want to know their "reasoning" for not wanting to use modules?

As for solutions, see these two threads:

• Add time
• time calculation