Of the four different syntax variants for the "link" command:
Link V[myDate] F[Date]
Link F[Date] V[myDate]
Link V[myDate] [Date]
Link [Date] V[myDate]

which is the correct one?

We generally recommend the use of an explicit F[...] in order to avoid ambiguities:

Link V[myDate] F[Date]
Link F[Date] V[myDate]

The difference between these two is the following:

The "Link" command triggers two actions, one before input and one after.

Before input, V[myDate] is changed in the first case above. It gets the value of the field F[Date].
In the second case above, the content of F[date] is changed; it gets the value of V[myDate].

After input, V[myDate] obtains the new value of F[Date] in both cases.