Relax UnixTime construction from String

[Beforerr]

Right now, we require that @assert length(s) == 29, I feel like that it is too strict and we could not parse some strings which are parsable for DateTime.

[ancapdev]

Agree, the reason being the DateTime parsing code is quite complex and not easily reusable without leaning on internals. If you'd like to PR a parsing that supports DateFormat I'd be happy to review and merge it.

This is the DateTime parsing code for reference: https://github.com/JuliaLang/julia/blob/v1.12.1/stdlib/Dates/src/parse.jl.

You may get away with parsing as DateTime and picking up the nanosecond portion separately.