I read an experience letter which said
"So and so" person has worked from "date1" till "date2".
Is it okay? Or should it be like this?
"So and so" person has worked from "date1" to "date2".
Either construction is fine.
In my mind using to is preferable, but until or till are acceptable alternatives.
There are examples in literature:
Till and until are synonyms in this context. They mean "up to a specific point in time". I would say that they are more likely to be found with simple (clock) times, rather than dates, but are not incorrect to use with dates.
I would say that you can use both, but that in some cases there can be a very slight difference in meanings regarding whether
Being Australian, I would consider "till" to be rather informal, but I believe that in American English it is perfectly acceptable as an alternative spelling to "until".