Oneself is used to describe someone doing something to themself. It is used to refer to the subject for emphasis on the subject acting alone. 

Here is an example:

"To achieve greatness includes putting oneself through difficult situations." In this sentence, oneself is expressing who is putting themself in difficult situations but unlike "themself" it is more vague. 

I'm not a teacher but I am a native English speaker, so if I didn't do a good job just comment or message me and I will try to clarify things!

Hi I'm Laura. I live in Colombia and I'm 15 years old. I love animals and I have two dogs, Nube and Lorenzo. I'm a student and I love art.
I am seeking friends and someone to help me learn English.

¡Muy bien! Hubo solo poco errores. Si practices mas, las oraciónes serán mas faciles. Estoy aprendiendo español. Tengo 16 años. ¡Espero que nos ayudaramos aprender las idiomas!

Very good! There were only little mistakes. If you practive more, the sentences will be very easy. I am learning Spanish. I'm 16 years old. I hope that we will help each other learn the languages!

<em>Today I got up very early and when I looked out of the window, I saw that it was raining outside. "What a day! "I thought," I want to study English at the library but I don't know what time the rain will stop." </em>
<em> Tomorrow That evening I asked a question to my friend who speaks English. </em>
<em> “Can you tell me how to pronounce 'Detectives'?" I asked. </em>
<em> "Detectives" he answered. </em>
<em> "His pronunciation very perfect!" I thought. I repeated his pronunciation several times however,</em>
<em>his pronunciation was different than mine.</em>

How do I say this correctly and naturally? Thank you.

Products have expiration dates.

<em>I don't know the context of this sentence so to me I think it would be better if you described the products. For example, "Dairy products have expiration dates." or "Cleaning products have expiration dates." Whatever you are discussing, I would include that in the sentence.</em>

The content could develop a change in colour and odor over time even if they didn't pass expiration dates.

<em>"The product could develop a change in color and/or odor despite not having passed the expiration date."</em>

The more recently a product is manufactured, the further away the expiration date is from the present. 

<em>This sentence sounds awkard, I would say "The expiration date depends on the date the product was manufactured, an older product will have a sooner expiration date than a newer product." </em>

If a product that was made today had three year's time to expire, the expiration date for that product would be 6/6/2017.

<em>"The product lasts three years before it will expire. If the product was made today, the expiration date would be June 6th, 2017." </em>

If there are two products that are made within 6 months gap of term, one after another, the latest product has a futher back expiration date.

<em>"If two products were made 6 months apart from each other, the first product would expire sooner."</em>

<em>Or, "If two products were made 6 months apart from each other, the second product would expire after the first."</em>

Dilemma

I am very excited because I have finally finished my semester so and now I will have more time to do things I enjoy. I would like to work in this summer but I don't want to miss the world cup. I have This is a big dilemma because I really love soccer.

¡Muy bueno! Pude comprender que digas, pero traté usar palabras mas naturales así que la oración sueña mejor. En todo, una buena oración.

Very nice! I was able to understand what you were saying, but I tried to use more natural words so that the sentence sounds better. Overall, a good sentence.