Hi
There are 2 options for entering dates in excel
1 - using a regional format
For ex. the date 21-1-2018 may be OK using the UK regional format, but the same date using the US regional format can be 1/21/2018 or using the German regional format is 21.1.2018.
If you want this option, check the regional format for your windows installation and use it when entering the date.
2 - using the international date format
In this case, use 2018-01-21.
This works everywhere
In my work (and play) with excel I always use the international date format, but you choose the one your like.