The ImageModificationTime appears to be handled both in terms of setting the contents of the tIME chunk [1] from standard metadata and for setting the value of the standard /Document/ImageModificationTime from the tIME chunk. The iTXt, tEXt, and zTXt chunks [2] however may contain the Creation Time keyword and corresponding time of original image creation but this time value does not appear to be used to set the standard /Document/ImageCreationTime nor does the standard /Document/ImageCreationTime appear to be used to set the value of the Creation Time in the text chunk(s).

[1] http://libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.tIME
[2] http://libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.Anc-text