I want to include current date in a heading.
I’m not exactly sure what you mean by “heading”. Assuming it’s any ungrouped drawing object accepting text content you can edit that text and >Insert>Field>Date (variable) the same way as you would do it in Writer.
According to @Lupp I suggest to insert the field in master page, upper part, above title place holder.
If you click into the field (date) for some times and then use properties (right mouse click) you can choose some different formats in reading …
I haven’t noticed any “Header” but it seems to exist (part of so called Master Elements :: Placeholder). It is greyed out, but not the other elements. There does exist a footer additional to other parts around the bottom of the slide/page. Probably it depends on the master slide’s design (?).
Used version: LO 6.2.0.3-64 on W7-64