A fixed point in time is literally called a ‘time crystal’ in physics.

Now that’s theory that developed after Dr Who put it out into the global conversation, much as Albucierre’s Warp theory and Star Trek. It doesn’t make it any less a completely valid science fiction explanation.

In terms of time pushing back, entropy comes into it. Pushing an essential event further down the time stream should be harder, but pushing it to an earlier date would be orders of magnitude harder. Eliminating the event altogether and forking the time steam into a new course will be much more difficult than either.

It can be argued that First Contact is classic example of time pushing back. While the Enterprise was able to ensure the first warp flight happened in the very narrow window, there were slippages in the details around the event regardless of the preservation of the date. The Prime Universe timeline was overwritten, just not as obviously as in a change of a calendar date.