Single day specification implemented on the server side in r10367 in svn://svn.zabbix.com/branches/dev/zbx-2055-interval-validation .
dev branch: svn://svn.zabbix.com/branches/dev/ZBX-2055
(1) The interval "1-2,00:00-24:00;3,00:00-21:00" fails validation with an error:
Incorrect flexible interval "3,00:00-21:00". [CItem.update -> CItem.checkInput -> CItemGeneral.checkInput -> CItemGeneral.validateDelayFlex]
<Vedmak> In items there should be only one interval per row. CLOSED
<richlv> let's document that - REOPENED 
(2) Let's rename CItemGeneral::validateDelayFlex to checkDelayFlex according to the guidelines we've discussed some time ago.
<Vedmak> RESOLVED
<pavels> CLOSED.
(3) Lets replace the validateDelayFlex() function with a validator class the way we discussed.
<Vedmak> RESOLVED
<pavels> Please, add the ability to pass additional options to the validator to avoid using different validation methods. CLOSED.
(4) Validation should be performed when adding the interval, not when saving the whole item.
<Vedmak> RESOLVED
<pavels> CLOSED.
TESTED.
Fixed in pre-2.0.2rc1 r28648, pre-2.1.0rc1 r28649
(5)
Cannot save item without flexible interval.
<pavels> CLOSED.
note that this is now a blocker for 2.0.2 because of the fix in 28661
TESTED once again.
Fixed in pre-2.0.2rc1 r28680, pre-2.1.0rc1 r28681
(6)
When I save an empty time period, I get the following error:
Uninitialized string offset: -1 [include/classes/validators/CTimePeriodValidator.php:40]
Incorrect time period "".
Plus, the error message should say "Incorrect working time" instead of "Incorrect time period."
<Vedmak> RESOLVED
<pavels> CLOSED. I've corrected the test in r28728.
Fixed in pre-2.0.2rc2 r28729, pre-2.1.0rc1 r28730