100 was a number.
At Rose Tyler's request, the Ninth Doctor took the TARDIS a hundred years through the Time Vortex from 2005 to 2105. However, the Doctor claimed that it was "a bit boring" and so they went even further into the future before stepping out of the TARDIS. (TV: The End of the World)
In maths, 258 minus 158 was 100. Rose Tyler answered this correctly in The Weakest Link. (TV: Bad Wolf)
As Ood Sigma revealed to the Tenth Doctor when he returned to the Ood Sphere, one hundred years had passed following his involvement in the Revolution of the Ood. (TV: The End of Time)
Sunday was 100 light-years from Earth, (PROSE: Wetworld) as was Beta Caprisis. (PROSE: Sanctuary)