Google Sheets – Get Halfway Date Between Two Dates

This will fix it for you:

=text(regexreplace("timestamp","^\w{3},",""),"MM/DD/YYYY")

you could also transform to remove the additional time parameter at the end with this:

=text(regexreplace(regexreplace("timestamp","^\w{3}, ",""), ", \d+:\d+\w+",""),"MM/DD/YYYY")

OR if your data consistently is formatted the way you did in your examples (with the commas present) you can simply do this:

=TEXT(REGEXEXTRACT(C10,",(.*),"),"MM/DD/YYYY")

You can use

=datevalue("01-"&B2)

(single-cell formula that can be copied down), or

=arrayformula(datevalue("01-"&B2:B))

(converts an entire column at once), or

=arrayformula(iferror(datevalue("01-"&B2:B)))

(does the same, while suppressing any error messages).

This works because the strings like 01-Oct-14 are understood by datevalue command correctly, at least in your locale.