Is it acceptable to submit old rec letters?

[evasive fish]

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I've been mentored by 4 PIs and will apply MSTP next cycle (2021-2022). I've been using Interfolio to store rec letters over the last few years.

By the time of application the letters will be:

Letter 1 (most significant research experience) -------- 2 years old
Letter 2 ---------------------------------------------------1 year old
Letter 3 ---------------------------------------------------- new
Letter 4 (current PI) --------------------------------------- new

Is this acceptable, or is it strictly imperative to have all letters written on the same year of app submission?
Thanks!

 

[DoctorWhere]

If you can get the first two letters refreshed, it is preferable. If not, then send in what you have. Letter 1 is important.