Full Timeline

Stan Lee

1.   The X-Men: Friday 31^st March to Saturday 1^st April, 1978.

2.   No-one Can Stop the Vanisher!: Monday 3^rd to Friday 7^th April, 1978.

3.   Beware of the Blob!: Saturday 22^nd April, 1978.

4.   The Brotherhood of Evil Mutants!: Sunday 23^rd April to Tuesday 2^nd May, 1978.

5.   Trapped: One X-Man!: Saturday 6^th to Sunday 7^th May, 1978.

6.   Submariner! Joins the Evil Mutants: Saturday 13^th to Monday 15^th May, 1978.

7.   Return of the Blob: Thursday 29^th to Friday 30^th of June, 1978.

8.   The Uncanny Threat of... Unus the Untouchable!: Saturday 1^st to Saturday 8^th July,
      1978.
9.   Enter: the Avengers: Saturday 15^th to Wednesday 19^th July, 1978.
10. The Coming of... Ka-Zar: Tuesday 15^th of August, 1978.
11. The Triumph of Magneto!: Tuesday 22^nd of August, 1978.
12. The Origin of Professor X!: Tuesday 22^nd of August, 1978.
13. Where Walks the Juggernaut!: Tuesday 22^nd of August, 1978.
14. Among us Stalk... the Sentinels!:  Tuesday 5^th to Wednesday 6^th September, 1978.
15. Prisoners of the Mysterious Master Mold!: Wednesday 6^th September, 1978.
16. The Supreme Sacrifice: Wednesday 6^th September, 1978.
17. ...And None Shall Survive!: Wednesday 6^th to Thursday 7^th September, 1978.
18. ...If Iceman Should Fail!: Thursday 7^th to Friday 8^th September, 1978.
19. Lo!  Now Shall Appear -- The Mimic! Wednesday 1^st to Thursday 2^nd November, 1978.

Lee's tenure: 31 months.  Time passed for X-Men: 7 months.  Compression constant: 1 Lee month = 4.42 standard months.

Roy Thomas

20. I, Lucifer...: Monday 6^th November, 1978.
21. From Whence Comes... Dominus?: Monday 6^th to Tuesday 7^th November, 1978.
22. Divided -- we Fall!: Thursday 31^st May to Friday 1^st June, 1979.
23. To Save a City: Friday 1^st June, 1979.
24. The Plague of... The Locust! Tuesday 14^th to Saturday 18^th September, 1979.
25. The Power and the Pendant! Monday 20^th to Thursday 23^rd September, 1979.
26. Holocaust! Thursday 23^rd to Friday 24^th September, 1979.
27. Re-Enter: The Mimic!: Sunday 26^th to Tuesday 28^th September, 1979.
28. Wail of the Banshee!: Sunday 1^st November, 1979.
29. When Titans Clash!: Saturday 5^th December, 1979.

Thomas's tenure to date: 9 months.  Time passed for X-Men: 13 months.  Compression constant: 1 Thomas month = 0.69 standard months.

A few words of explanation.  We know issues #25 at least starts in Summer, and takes place immediately after issue #24, hence the dates above.  Issue #27 takes place in Autumn, and we need to include enough time for Angel to at least have partialyl recovered. Since the difference between issues #26 and #28 is now about two months, we can put issue #26 in the middle.  This has the knock-on effect of making Beast's estimated birth date now the second of November, 1959, rather than the twenty-fourth of September.