I always knew to shorten the hours should use only the "h", never “h” together with “s” (hs) or the “rs” (hrs), but searching the Internet I found an interesting article which further cleared that matter.
I found the article hit the stands for hours that explains it and alerted me to another point, nay, the two points that always insisted on putting.
According to Article, the writing of hours using the colon to separate the time of minutes (ex.: 07:30) It is wrong and the correct, for our Portuguese, would use letter “h” for this function (ex.: 07h30).
Since then I never wrote emails using the colon to separate the time of minutes or seconds. To prefer to find that is much more readable.
Another lesson learned and I have helped.