Sreev,
You can't apply names in the Symbol Table to LB, LW or LD addreses since they have no existence outside the block in which they are referenced. These addresses are 'Local' bytes, words, or doubles and are simply methods of addressing the same stack of memory which is created dynamically for the TEMP declarations of every block. When the block is called, whether it is an OB called by the operating system, or an FC or an FB, an area of memory is assigned to hold these TEMPs. As soon as the block completes and control exits this piece of memory is simply unallocated and then can be assigned to the next block that requires it.
Be aware : the operating system leaves the contents of this memory unchanged when it unallocates it. So never make assumptions about what is in these registers when a block starts executing. Write to them before reading them is a good rule. In the case of OB1 variables, the operating system writes to them as soon as it starts executing so you can rely on these data and read them immediately.
If you want to use symbolic addressing you have to stick to the datatype used for declaring the TEMPs. OB1_DATE_TIME is declared as a DATE_AND_TIME variable, so you could use the symbolic name #OB1_DATE_TIME for it. However, if you want to access individual bits or bytes within this variable you end up using direct LB addressing etc.
Regards
Ken