I will let you liturgical experts and interested dabblers provide this good reader with some help.
I may be missing something, but I can’t see much of a problem. You readers might see differently.
From a reader:
A question for you and your readers, which is bound to arise as more and more places offer the Triduum in the EF. Our local clergy here have so far welcomed the idea of a celebration of the entire Triduum in the extraordinary form, in addition to the parish’s celebration in the ordinary form. Amazing! Who would have thought it possible a few years ago?
One practical difficulty we have run into is a conflict between different books as regards the permissible range of times for the Holy Thursday and Good Friday liturgies — a question, naturally, that has to be solved if there will be two of each. The discrepancy can be seen in comparing the Sacred Congregation of Rites General Decree from November 16, 1955 (printed as an appendix in Fr. McManus’s The Rites of Holy Week, 1956) and the fine print in the Liber Hebdomadae Sanctae Cantus Gregoriani (PCP reprint, 2011, of a book that appears to be a Desclee publication of around 1956).
SCR General Decree: “On Holy Thursday … the Mass of the Lord’s Supper must be celebrated in the evening, at the most suitable hour; not, however, before 5 nor after 8 p.m.”
Liber Hebdomadae Sanctae: “The Mass must be celebrated at a convenient time in the evening, but not before 4 p.m. or after 9 p.m.”
SCR Decree: “On Good Friday, the solemn liturgical service is celebrated in the afternoon, and indeed about 3 p.m.; but if a pastoral reason urges this, a later hour may be chosen — not, however, beyond 6 p.m.”
Liber Hebdomadae Sanctae: “The solemn Liturgy is celebrated in the afternoon, about 3 p.m.; however, for pastoral reasons, it may begin earlier, from mid-day onwards, or at a latest hour, but not after 9 o’clock.”
There is no discrepancy as regards the Paschal Vigil.
The SCR decree, which appears to give more restrictive times, is the later law. It is more authoritative than some book.
If this is truly an issue, stick to the more restrictive times: 5-8 for Holy Thursday, “about” 3 – 6 for Good Friday.
If there is real doubt, you are free to submit the question to the Pont. Comm. Ecclesia Dei. As long as the doubt remains unanswered, you are free to utilize the more expansive time frames.