Hey Bhawna,
Im not sure how much detail you are looking for here but i think you are looking for either one of these 2 answers:
1) Technically: A date is updated in a sales order thanks to its copy controls allowing you to relate a previous document into a sales order. In case no reference document is used, then you would be looking at just adopting a date from master data entered previously which is loaded via the program or modules that are masked behind your interface.
2) If you want to know about a service having an automatic date and are wondering why this is, then have a look at your service master (basically your service catalogs, etc), or have a look at your contracts. Maybe you have called up a contract (for example a a contract for a predefined service end date) in your sales order.
Hope this helps,
Alex Wardrop