Textastic supports TextExpander.  TextExpander snippet expansion can do dates.  It is expensive, you have to pay by the month or by the year.

https://feedback.textasticapp.com/en/communities/1/topics/1507-user-definable-code-macro-buttons

http://robertmarkbramprogrammer.blogspot.com/2011/08/text-macros-in-ios-insert-date-on-ipad.html?m=1

The timestamp-keyboard costs less and has fewer features, but can also do dates and times

https://www.cambriancreative.com/timestamp-keyboard