[fpc-pascal] Parse strings like "day-1" to convert in a time stamp

[Michael Van Canneyt]

> On Sun, Oct 14, 2018 at 8:19 PM Michael Van Canneyt
> <michael at freepascal.org> wrote:
>>
>>
>>
>> On Mon, 15 Oct 2018, Sven Barth via fpc-pascal wrote:
>>
>> > Marcos Douglas B. Santos <md at delfire.net> schrieb am So., 14. Okt.
>> 2018,
>> > 16:15:
>> >
>> >> Do you know any Pascal lib to parse strings to convert in date/time
>> values?
>> >> The user is supposed to type human-friendly values like:
>> >> - "day"    => trunc(now)
>> >> - "day-2" => trunc(now-2)
>> >> - "week" => trunc(now-7)
>> >> - "yesterday" => trunc(now-1)
>> >> - etc...
>> >> But they could type even their computer date format or ISO as well.
>> >>
>> >> A lib in C, but very complex:
>> >> - https://github.com/gagern/gnulib/blob/master/lib/parse-datetime.y
>> >>
>> >> I would like to start with something simpler, if possible. Thank you.
>> >>
>> >
>> > I don't know a specific library that fulfills your needs, but maybe
>> you can
>> > build something atop of FpExprPars:
>> > http://wiki.lazarus.freepascal.org/How_To_Use_TFPExpressionParser
>> > At least according to the source it also supports datetime handling.
>
> Thanks, Sven.
>
>>
>> It does, it's used (amongst other things) in the reporting engine;
>
> Michael,
> But does it work using just date values - accordingly with date format
> in OS - or also with "day", "tomorrow", etc?
> If not, can I extend those classes without change the original ones?

All depends on the exact format you want to use.
day -1
can be easily implemented by registering a function day, which returns teh
same Date()

double quotes are supported as delimiters for identifiers, so "Day" should
work, but a date literal is going to be tricky.

Michael.