Chronophobia is earned during Chapter 19 "Once More, With Feelings!" after completing all the other achievements in the game. Load the chapter and if you had previously collected all the Luxomails and continued to Chapter 20 without returning to the main menu, the the only Luxomails you should need to collect are the last 4 in the area near the rocket ship. Once those last Luxomails are recollected Ben and Dan will comment about collecting them all.
Proceed toward the altar where Dan sacrificed Ben in an earlier chapter. Before you reach the altar there is a doorway labeled 'Door?':https://imgur.com/SlJYTb1
Have Ben 'Use' the door to bring up a 5 digit combination lock.
The combination is 63527.
Once you enter the doorway the achievement will unlock. If you have not collected all the Luxomails you can still enter the combination, but you will be not be allowed through until all the Luxomails are collected.
The combination is found by solving the riddle of the in-game 'Achievements' menu.
Completed Achievements diagram found by earning all other achievements except for 'Basically Read a Book':https://imgur.com/2xn83rf
Path through the Achievements diagram found on the wall of the stairs the exploding barrel rolls down.https://imgur.com/VH9Ohu1
Operands through the Achievements diagram found on the wall in the records room.https://imgur.com/douQxUu