Have woven labels made and you do not have to worry about the inks used, and if they will fade. On the last line have a date code woven in. I use:
10JFMAMJJASOND11. You could use 10 1234 11 the year, 1st, 2nd, 3rd, 4th quarter. 10WSSF11 the year Winter, Spring, Summer, Fall. Use whatever code makes sense to you, there is no required standard.
When I complete something, I take a stitch under the year and month with floss to date the garment. Works for me, and is no sweat.