What is the proper way to express date durations?

Question

I am working on a web site that is internationalized for English and Mexican Spanish. The site displays messages to users when they do something wrong on a form (i.e. validation messages). If they enter a date that is outside of a valid date range, the web site creates a message like the following (using English or Spanish based on the user's language preference):

Date must occur on or between 05/21/2017 and 06/20/2017

La fecha debe ocurrir en o entre 05/21/2017 y 06/20/2017

I'm not sure the phrase the site builds for Spanish is correct. I have the following questions:

• Is the date format correct for a Mexican Spanish speaker? I am aware of the Spanish way of expressing dates (i.e. day first) but since Mexican Spanish speakers deal with American English so frequently, I'm not sure. Should the dates be expressed as "21/05/2017" and "20/06/2017"?
• Should "el" precede both dates?
• Is it better to say "debe tener lugar entre" instead of "debe ocurrir en o entre"? Valid dates include the start and end dates in the range.

Answer 1

Is the date format correct for a Mexican Spanish speaker?

Grammatically speaking, it is recommended that the form dd/mm/yyyy is used in every Spanish-speaking country. According to Wikipedia, Mexico does follow said format. I guess most Mexicans would find any other format wrong.

This doesn't mean that a mm/dd/yyyy wouldn't be understood. It probably would... as long as the day is past the 12th of the month (05/20/2017 is clear, 05/08/2017 not so much). So better stick to the Spanish format.

Should "el" precede both dates?

That would be more correct, yes.

Is it better to say "debe tener lugar entre" instead of "debe ocurrir en o entre"?

Not really. They mean about the same.