Convert only non-empty datetime from client to UTC

issue3866
review5261002
(grafted from 20c4b6c559e5f7f20405dc39d99f3e61c6c6bab5)

--HG--
branch : 3.2
extra : source : 20c4b6c559e5f7f20405dc39d99f3e61c6c6bab5
Prepare release 3.2.1

--HG--
branch : 3.2
Added tag 3.2.1 for changeset 41c2b1119e77

--HG--
branch : 3.2
Increase version number

--HG--
branch : 3.2