  The double or extended main-sequence turnoffs (MSTOs) in the color-magnitude
diagram (CMD) of intermediate-age massive star clusters in the Large Magellanic
Cloud are generally interpreted as age spreads of a few hundred Myr. However,
such age spreads do not exist in younger clusters (i.e., 40-300 Myr), which
challenges this interpretation. The effects of rotation on the MSTOs of star
clusters have been studied in previous works, but the results obtained are
conflicting. Compared with previous works, we consider the effects of rotation
on the MS lifetime of stars. Our calculations show that rotating models have a
fainter and redder MSTO with respect to non-rotating counterparts with ages
between about 0.8 and 2.2 Gyr, but have a brighter and bluer MSTO when age is
larger than 2.4 Gyr. The spread of the MSTO caused by a typical rotation rate
is equivalent to the effect of an age spread of about 200 Myr. Rotation could
lead to the double or extended MSTOs in the CMD of the star clusters with ages
between about 0.8 and 2.2 Gyr. However, the extension is not significant; and
it does not even exist in younger clusters. If the efficiency of the mixing
were high enough, the effects of the mixing would counteract the effect of the
centrifugal support in the late stage of evolution; and the rotationally
induced extension would disappear in the old intermediate-age star clusters;
but younger clusters would have an extended MSTO. Moreover, the effects of
rotation might aid in understanding the formation of some "multiple
populations" in globular clusters.
