Crystallized time

Periods of crystallized time were rare stretches of immutable time, possessing huge inertia and making them difficult to tweak and disrupt. Any disruptions or changes to events would send ripples of unimaginable power out through the substrata of crystallised time and into the Time Vortex, creating anomalies, warps in time; breakthroughs from an alternative universe where what happened because of the change — happened. This usually manifested as small things like dates changing their days or phantom newspaper headlines, before time sprang back into its natural shape, releasing huge amounts of energy.

The Garvond and Epsilon Delta planned to disrupt a thousand-year period of cystallized time in Earth's history, centring on an assassination that never happened in 1993 and the Garvond growing its power in the 24th century. Epsilon Delta planned to trigger the assassination to release enough temporal energy to restore his TARDIS and give him the power to get revenge on the Time Lords who had ignored him and shunted him to the side. (PROSE: The Dimension Riders)

When contemplating his entire life after his first encounter with the Silurians, the Seventh Doctor considered if he had created a parallel universe with Ace's Earth and all its subsequently crystallized points of future history in which he was still alive. He abandoned the idea after realising he'd travelled into crystallised moments of the universe's future from a point before his supposed death by the Silurians. (PROSE: Blood Heat)

When Tegan asked the Fifth Doctor about preventing Nyssa's mummification, he stated they probably couldn't: "Time is already set in it's course, crystallised along a particular web-way." As Nyssa had already been mummified before they travelled back in time to try and prevent it, the Web of Time had already crystallised at that node, meaning it might as well have been "set in stone" as the two time zones had to stay relative do to temporal dynamics. (PROSE: The Sands of Time)