According to the code of the png coder you should be able to do this (I have not tested it):
I also found a todo about reading tIME chuncks. At this moment there is not support for it but I might be able to add support for it in the next release of Magick.NET. Why do you need to set a tIME chunk?
input.SetDefine(MagickFormat.Png, "exclude-chunk", "text");