I don't know. Every source seems to make a different claim. I have childcare books that claim the heart begins beating between 5-6 weeks of pregnancy... which would fall around a month and a half after conception. In fact, the booklet from which I take that number contains the very photos in the article that started this thread. Even at 15 days, though, it's hard to argue against a heartbeat.
http://www.visembryo.com/baby/index.html
Keeping in mind that the physician's timeline runs from last assumed day of ovulation, the following is cited:
19 - 21 days post-ovulation; stage 9
If you could look at the embryo from a top view, it would resemble the sole of a shoe with the head end wider than the tail end, and a slightly narrowed middle.
Somites, which are condensations composed of mesoderm, appear on either side of the neural groove. The first pair of somites appear at the tail and progress to the middle. One to three pairs of somites are present by Stage 9. Every ridge, bump and recess now indicates cellular differentiation.
A head fold rises on either side of the primitive streak. The primitive streak now runs between one-fourth to one-third of the length of the embryo.
Secondary blood vessels now appear in the chorion/placenta. Hematopoietic cells appear on the yolk sac simultaneously with endothelial cells that will form blood vessels for the newly emerging blood cells.
Endocardial (muscle) cells begin to fuse and form into the early embryo's two heart tubes.
21 - 23 days post-ovulation; stage 10
Stage 10 reflects rapid growth and change as the embryo becomes longer and the yolk sac expands.
On each side of the neural tube, between four and twelve pairs of somites can exist by the end of Stage 10. The cells which become the eyes appear as thickened circles just off of the neural folds. The cells of the ears are also present.
Neural folds are rising and fusing at several points along the length of the neural tube concomitant with the budding somites which appear to "zipper" the neural tube closed. Neural crest cells will eventually contribute to the skull and face of the embryo.
The two endocardial tubes formed in Stage 9 fuse in Stage 10 to form one single tube derived from the roof of the nueral tube, which becomes S-shaped and makes the primitive heart asymmetric. As the S-shape forms, cardiac muscle contraction begins.
Now you can see why many say abortion stops a beating heart.