date parsing ignores missing mandatory parts, parses invalid dates

[tpapp]

Eg in

julia> VERSION
v"0.7.0-beta.275"

julia> using Dates

julia> tryparse(Date, "19800101", dateformat"yyyy-mm-dd")
19800101-01-01

I would expect nothing, since the -mm-dd part is missing.

(discussion here)

[anowacki]

I have also had a bad experience using tryparse with Dates, Times and DateTimes.

Specifically, I was parsing data in which a date was unexpectedly absent and therefore gave an empty string. It turns out that when given "", Date and Time constructors return "0001-01-01" and "00:00:00" respectively. tryparse(Date, "") and tryparse(Time, "") give the same results. Similarly to Tamas and others in the Discourse discussion linked, I would have expected nothing for tryparse since the format does not match the default format. (default_format(Date) is dateformat"yyyy-mm-dd", whilst default_format(Time) is dateformat"HH:MM:SS.s".).

As a consequence of the above, I guess, tryparse(DateTime, "") returns "0001-01-01T00:00:00", not nothing as I would have expected. However as another inconsistency, parse(DateTime, "") does actually throw an ArgumentError("Invalid DateTime string"), which is helpful.

I like the present flexibility of the constructors, but I believe that if passed a DateFormat, parse and tryparse should be strict, and I agree with others that there is no need for the constructors and parsing functions to have the same 'strictness policy'.

At the very least, I think it should be documented clearly somewhere that currently empty strings are valid Dates, Times and DateTimes (and I'm happy to get a PR ready I have submitted a PR to that effect).