Island (TV)

Both are based on highly rated visual novels where the protagonist suddenly gets dragged in many mysteries while surrounded by beautiful women. Furthermore these anime are very similar because of time travel (explaining more would be spoilers) 

Both deal with time travel Both have good story and plot Both good drama Both have likable casts Both have a cute heroine 

Similar theme of repeating timelines and mystery 

both have the same concept of going back in time to save someone. Both the female protagonist also has white hair and the storyline is almost the same but has slight changes. 

Both anime are about - main boy spawning on an island over and over to save it - searching for the main heorine  

It involves them harem girls with issues against the real world and a mysterious guy from outta nowhere comes in and helps them.  

both have the same time traveling theme in it, that seemly defines Einstein's theory of relativity. Except one has a mystery genre while the other has an action genre.  

The usual Time travel in order to meet someone that seemly defies the Einstein's Theory of Relativity.  

Both animes follow MCs in their mission to uncover what truly transpired and prevent the death of their precious one while protecting those they care about in the present. 

Both have the same kind of drama, fantasy and time travel that somehow violates Einstein theory of relativity.  

Sometimes, things run deeper than what they appear to be. A time traveler whose mission is to save a girl? A person who sees flags that determine the fate of the people around him? Both seem to be shallow at first glance, but that is because something else runs beneath them...  

The secrets that is confined between both series is quite a bit reminiscent of each other. Granted, these secrets can truly be understood once one completed such long journeys. 

Similar female characters around the main lead with time travel aspect in both. Both also have that kind of drama that comes with these types of anime.  

Have the same theory about time-travel about the future and bit tragic  

To those seeing this recommendation, I ask thee to be wary as this recommendation is better seen when you have at least completed one of the anime being recommended. Time is truly a mighty adversary to those who dare violate its principles, and the results might be either sweet or catastrophic should one succeed or fail. The nostalgic concept of time is differently portrayed in each series, but essence remains unchanged. Both series try to challenge the flow of time and yield results that could regarded as the opposite of the other. 

Promises, promises, promises. Perhaps the greatest and the worst thing regarding the common promise of "I shall return" is that time is truly limited and humans will inevitably die of old age. Kono Aozora ni Yakusoku wo deals with the pain of having to lose friends, but it always has that sliver of hope that you will meet them again. In Island... well, it is much, much, more.  

Also based on a visual novel that explores the "routes"/stories of three different cute girls. Takes place in the summer with beach scenes. Gorgeous animation. Also deals with reincarnation. Air is a million times better - you will actually like the characters and feel invested in the story. 

Oh, the two share such poetic premise of a seemingly lost man fatefully meeting a girl in a place that represents their entire relationship: one a clock tower, the other in an island shoreline. While the two have different settings, one in wartime and the other in modern era, the development is surely reminiscent to each other. Moreover, both share the prospect of romance that is bound to brave even the harshest waves of time and coincidence. 

They say that love can topple mountains, destroy towns, flood cities, and brave the tests of time. It's really astonishing to witness what the bond between two people can do and both these series portray such imagery that it's very perfect!