Tabris's answer still applies. Most places I've seen/talked to state they use the beginning of the month.
In your case, if the date on the tank is 4 @ 09, then it would have needed to be hydroed by 3/31/14. Come 4/1/14, it would be a no go since there is no way to know if the tank was made on the 1st or the last day of the month.
Graphite Dangerous Power REV-I
– TechT bolt engine, REV-I & Disruptive barrel kits, Prophecy V2 w/ Empire feedgate, Crossfire SS 68/4500
Maroon and Polished Aluminum Dangerous Power Threshold
– 7th Element board, TechT bolt engine, magnet trigger kit, REV-I PMD, Sly Dual Carbon barrel kit, Prophecy V2, Crossfire SS 68/4500
Black and Silver Dangerous Power F8
– APE Rampage board, VS bolt, magnet trigger kit, F8 gauge, REDZ barrel kit, Halo B w/ Exalt feedgate, PMI 68/4500
Polished Aluminum and Red Dangerous Power F8
– Virtue board, VS bolt, magnet trigger kit, Halo B w/ Exalt feedgate, PMI 68/4500