The ACF plugin will save in a format you give it. If you enter the “Save Format” (Darstellungs-format I believe in german) field as “d.m.y” then it will be stored in a format “1.12.16” for date 1st of december 2016. I just tested this and the database records confirm. In this case if you search “1.12” or “12.16” (or any substring of it) it will match the date, as it’s treated as a string.
Also, make sure to choose the date again and save the post once more, as ACF will not convert the fields automatically which are already in the database.
If I may recommend a more widely used format might be “dd.mm.yy”, which includes the full year and leading zeros as well and it looks like “01.12.2016” in the database, and might be much easier to search for as well.Best,
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