Hello, and thanks for the interest in the mod!
The answer to your question about editing the date of alien invasion is technically "yes", but it's not so simple. Code-wise, there is no "invasion date"; instead, all events in the mod which are limited by time passed refer to a number of months passed since the beginning. If you check missionScripts_XCOMFILES.rul, you see for example:
- type: alienFlyby
missionWeights:
0:
STR_ALIEN_FLYBY: 100
firstMonth: 25
executionOdds: 8
startDelay: 20
randomDelay: 43500
This means that the mission STR_ALIEN_FLYBY starts happening at the earliest in month 25 (beginning from 0), which is January 1999.
In order to postpone the alien invasion by, say, one year, you would have to increase all relevant instances of firstMonth by 12. (Be careful to not affect other missions, like cult activities, or you may end up having no missions at all for a year.) As a bonus, you can also open vars_XCOMFILES.rul and change startingTime from 30th of December 1996 to 30th of December 1995 (don 't forget to change the weekday as well to match the calendar), to ensure that the invasion still happens January 1999, but you get an extra year.
Having said all that, I don't think it's that worth it - the invasion is not a complete game changer and you should be fine going at your own speed regardless.