Switch to UnixMilli funcs (Go 1.17)

[vtopc]

https://pkg.go.dev/time#Time.UnixMilli
and
https://pkg.go.dev/time#UnixMilli

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Get rid of https://pkg.go.dev/time#Time.UnixNano

The result is undefined if the Unix time in nanoseconds cannot be represented by an int64 (a date before the year 1678 or after 2262). Note that this means the result of calling UnixNano on the zero Time is undefined.