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None of the mentions defines a "Julian proleptic calendar".
Properly speaking, the Julian proleptic calendar would be the extension of the Julian calendar to dates before January 1, 45 B.C.E. when it came into effect. But I agree that "Gregorian proleptic calendar" must be what is meant.
Note that there is no single point for the Julian-to-Gregorian transition. Here's a table:
1582 France (most areas), Italy, Poland, Portugal, Spain (10 days)
1583 Austria, Germany (Catholic states) (10 days)
1587 Hungary (10 days)
1610 Germany (Prussia) (10 days)
1700 Germany (Protestant areas), Switzerland (Protestant areas) (10 days)
1752 United States (most areas), Canada (most areas),
United Kingdom (and colonies) (11 days)
1872/1873 Japan (12 days)
1916 Bulgaria (13 days)
1918 Estonia, Russia (13 days)
1923 Greece (13 days)
1926/1927 Turkey (13 days)
Other parts of the world underwent more complex transitions. ÂIn Sweden, the plan was to omit all leap days from 1700 to 1740 to provide a smooth transition from Julian to Gregorian, but the 1704 and 1708 leap days were not omitted, due to the pressure of war. Therefore, in 1712 an additional leap day, February 30, was added, undoing the 1700 leap day and leaving Sweden back on the Julian calendar. It eventually transitioned to the Gregorian calendar in 1753.