When working with dates, PHP uses the system time by default, which is dependent on the timezone the server is in. So, instead of doing tricky math with the results of the date function, you can simply change the timezone.
Here is the function used to change the timezone:
<?php
putenv("TZ=US/Detroit");
?>
One thing to keep in mind is that your MySQL will still be using the system time.
Here is a list of the timezone formats accepted in the above function:
AD +4230+00131 Europe/Andorra AE +2518+05518 Asia/Dubai AF +3431+06912 Asia/Kabul AG +1703-06148 America/Antigua AI +1812-06304 America/Anguilla AL +4120+01950 Europe/Tirane AM +4011+04430 Asia/Yerevan AN +1211-06900 America/Curacao AO -0848+01314 Africa/Luanda AQ -7750+16636 Antarctica/McMurdo AQ -9000+00000 Antarctica/South_Pole AQ -6448-06406 Antarctica/Palmer AQ -6736+06253 Antarctica/Mawson AQ -6835+07758 Antarctica/Davis AQ -6617+11031 Antarctica/Casey AQ -6640+14001 Antarctica/DumontDUrville AR -3436-05827 America/Buenos_Aires AR -3257-06040 America/Rosario AR -3124-06411 America/Cordoba AR -2411-06518 America/Jujuy AR -2828-06547 America/Catamarca AR -3253-06849 America/Mendoza Mendoza AS -1416-17042 Pacific/Pago_Pago AT +4813+01620 Europe/Vienna AU -3133+15905 Australia/Lord_Howe AU -4253+14719 Australia/Hobart AU -3749+14458 Australia/Melbourne AU -3352+15113 Australia/Sydney AU -3157+14127 Australia/Broken_Hill AU -2728+15302 Australia/Brisbane AU -2016+14900 Australia/Lindeman AU -3455+13835 Australia/Adelaide AU -1228+13050 Australia/Darwin AU -3157+11551 Australia/Perth AW +1230-06858 America/Aruba AZ +4023+04951 Asia/Baku BA +4352+01825 Europe/Sarajevo BB +1306-05937 America/Barbados BD +2343+09025 Asia/Dacca BE +5050+00420 Europe/Brussels BF +1222-00131 Africa/Ouagadougou BG +4241+02319 Europe/Sofia BH +2623+05035 Asia/Bahrain BI -0323+02922 Africa/Bujumbura BJ +0629+00237 Africa/Porto-Novo BM +3217-06446 Atlantic/Bermuda BN +0456+11455 Asia/Brunei BO -1630-06809 America/La_Paz BR -0351-03225 America/Noronha BR -0127-04829 America/Belem BR -0343-03830 America/Fortaleza BR -0712-04812 America/Araguaina BR -0940-03543 America/Maceio BR -2332-04637 America/Sao_Paulo BR -1535-05605 America/Cuiaba BR -0846-06354 America/Porto_Velho BR -0308-06001 America/Manaus BR -0934-06731 America/Porto_Acre BS +2505-07721 America/Nassau BT +2728+08939 Asia/Thimbu BW -2545+02555 Africa/Gaborone BY +5354+02734 Europe/Minsk BZ +1730-08812 America/Belize CA +4734-05243 America/St_Johns CA +4439-06336 America/Halifax CA +4612-05957 America/Glace_Bay CA +5320-06025 America/Goose_Bay CA +6608-06544 America/Pangnirtung CA +4531-07334 America/Montreal CA +4901-08816 America/Nipigon CA +4823-08915 America/Thunder_Bay CA +6344-06828 America/Iqaluit CA +4953-09709 America/Winnipeg CA +4843-09429 America/Rainy_River CA +6245-09210 America/Rankin_Inlet CA +5024-10439 America/Regina CA +5017-10750 America/Swift_Current CA +5333-11328 America/Edmonton CA +6227-11421 America/Yellowknife CA +6825-11330 America/Inuvik CA +5946-12014 America/Dawson_Creek CA +4916-12307 America/Vancouver CA +6043-13503 America/Whitehorse CA +6404-13925 America/Dawson CC -1210+09655 Indian/Cocos CD -0418+01518 Africa/Kinshasa CD -1140+02728 Africa/Lubumbashi CF +0422+01835 Africa/Bangui CG -0416+01517 Africa/Brazzaville CH +4723+00832 Europe/Zurich CI +0519-00402 Africa/Abidjan CK -2114-15946 Pacific/Rarotonga CL -3327-07040 America/Santiago CL -2710-10927 Pacific/Easter CM +0403+00942 Africa/Douala CN +4545+12641 Asia/Harbin CN +3114+12128 Asia/Shanghai CN +2217+11409 Asia/Hong_Kong CN +2934+10635 Asia/Chungking CN +4348+08735 Asia/Urumqi CN +3929+07559 Asia/Kashgar CO +0436-07405 America/Bogota CR +0956-08405 America/Costa_Rica CU +2308-08222 America/Havana CV +1455-02331 Atlantic/Cape_Verde CX -1025+10543 Indian/Christmas CY +3510+03322 Asia/Nicosia CZ +5005+01426 Europe/Prague DE +5230+01322 Europe/Berlin DJ +1136+04309 Africa/Djibouti DK +5540+01235 Europe/Copenhagen DM +1518-06124 America/Dominica DO +1828-06954 America/Santo_Domingo DZ +3647+00303 Africa/Algiers EC -0210-07950 America/Guayaquil EC -0054-08936 Pacific/Galapagos EE +5925+02445 Europe/Tallinn EG +3003+03115 Africa/Cairo EH +2709-01312 Africa/El_Aaiun ER +1520+03853 Africa/Asmera ES +4024-00341 Europe/Madrid ES +3553-00519 Africa/Ceuta ES +2806-01524 Atlantic/Canary ET +0902+03842 Africa/Addis_Ababa FI +6010+02458 Europe/Helsinki FJ -1808+17825 Pacific/Fiji FK -5142-05751 Atlantic/Stanley FM +0931+13808 Pacific/Yap FM +0725+15147 Pacific/Truk FM +0658+15813 Pacific/Ponape FM +0519+16259 Pacific/Kosrae FO +6201-00646 Atlantic/Faeroe FR +4852+00220 Europe/Paris GA +0023+00927 Africa/Libreville GB +512830-0001845 Europe/London GB +5435-00555 Europe/Belfast GD +1203-06145 America/Grenada GE +4143+04449 Asia/Tbilisi GF +0456-05220 America/Cayenne GH +0533-00013 Africa/Accra GI +3608-00521 Europe/Gibraltar GL +7030-02215 America/Scoresbysund GL +6411-05144 America/Godthab GL +7634-06847 America/Thule GM +1328-01639 Africa/Banjul GN +0931-01343 Africa/Conakry GP +1614-06132 America/Guadeloupe GQ +0345+00847 Africa/Malabo GR +3758+02343 Europe/Athens GS -5416-03632 Atlantic/South_Georgia GT +1438-09031 America/Guatemala GU +1328+14445 Pacific/Guam GW +1151-01535 Africa/Bissau GY +0648-05810 America/Guyana HN +1406-08713 America/Tegucigalpa HR +4548+01558 Europe/Zagreb HT +1832-07220 America/Port-au-Prince HU +4730+01905 Europe/Budapest ID -0610+10648 Asia/Jakarta ID -0507+11924 Asia/Ujung_Pandang ID -0232+14042 Asia/Jayapura IE +5320-00615 Europe/Dublin IL +3146+03514 Asia/Jerusalem IN +2232+08822 Asia/Calcutta IO -0720+07225 Indian/Chagos IQ +3321+04425 Asia/Baghdad IR +3540+05126 Asia/Tehran IS +6409-02151 Atlantic/Reykjavik IT +4154+01229 Europe/Rome JM +1800-07648 America/Jamaica JO +3157+03556 Asia/Amman JP +353916+1394441 Asia/Tokyo KE -0117+03649 Africa/Nairobi KG +4254+07436 Asia/Bishkek KH +1133+10455 Asia/Phnom_Penh KI +0125+17300 Pacific/Tarawa KI -0308-17105 Pacific/Enderbury KI +0152-15720 Pacific/Kiritimati KM -1141+04316 Indian/Comoro KN +1718-06243 America/St_Kitts KP +3901+12545 Asia/Pyongyang KR +3733+12658 Asia/Seoul KW +2920+04759 Asia/Kuwait KY +1918-08123 America/Cayman KZ +4315+07657 Asia/Almaty KZ +5017+05710 Asia/Aqtobe KZ +4431+05016 Asia/Aqtau LA +1758+10236 Asia/Vientiane LB +3353+03530 Asia/Beirut LC +1401-06100 America/St_Lucia LI +4709+00931 Europe/Vaduz LK +0656+07951 Asia/Colombo LR +0618-01047 Africa/Monrovia LS -2928+02730 Africa/Maseru LT +5441+02519 Europe/Vilnius LU +4936+00609 Europe/Luxembourg LV +5657+02406 Europe/Riga LY +3254+01311 Africa/Tripoli MA +3339-00735 Africa/Casablanca MC +4342+00723 Europe/Monaco MD +4700+02850 Europe/Chisinau MG -1855+04731 Indian/Antananarivo MH +0709+17112 Pacific/Majuro MH +0905+16720 Pacific/Kwajalein MK +4159+02126 Europe/Skopje ML +1239-00800 Africa/Bamako ML +1446-00301 Africa/Timbuktu MM +1647+09610 Asia/Rangoon MN +4755+10653 Asia/Ulan_Bator MO +2214+11335 Asia/Macao MP +1512+14545 Pacific/Saipan MQ +1436-06105 America/Martinique MR +1806-01557 Africa/Nouakchott MS +1644-06213 America/Montserrat MT +3554+01431 Europe/Malta MU -2010+05730 Indian/Mauritius MV +0410+07330 Indian/Maldives MW -1547+03500 Africa/Blantyre MX +2105-08646 America/Cancun MX +1924-09909 America/Mexico_City MX +2313-10625 America/Mazatlan MX +2838-10605 America/Chihuahua MX +3152-11637 America/Ensenada MX +3232-11701 America/Tijuana MY +0310+10142 Asia/Kuala_Lumpur MY +0133+11020 Asia/Kuching MZ -2558+03235 Africa/Maputo NA -2234+01706 Africa/Windhoek NC -2216+16530 Pacific/Noumea NE +1331+00207 Africa/Niamey NF -2903+16758 Pacific/Norfolk NG +0627+00324 Africa/Lagos NI +1209-08617 America/Managua NL +5222+00454 Europe/Amsterdam NO +5955+01045 Europe/Oslo NP +2743+08519 Asia/Katmandu NR -0031+16655 Pacific/Nauru NU -1901+16955 Pacific/Niue NZ -3652+17446 Pacific/Auckland NZ -4355-17630 Pacific/Chatham OM +2336+05835 Asia/Muscat PA +0858-07932 America/Panama PE -1203-07703 America/Lima PF -1732-14934 Pacific/Tahiti PF -0900-13930 Pacific/Marquesas PF -2308-13457 Pacific/Gambier PG -0930+14710 Pacific/Port_Moresby PH +1435+12100 Asia/Manila PK +2452+06703 Asia/Karachi PL +5215+02100 Europe/Warsaw PM +4703-05620 America/Miquelon PN -2504-13005 Pacific/Pitcairn PR +182806-0660622 America/Puerto_Rico PS +3130+03428 Asia/Gaza PT +3843-00908 Europe/Lisbon PT +3238-01654 Atlantic/Madeira PT +3744-02540 Atlantic/Azores PW +0720+13429 Pacific/Palau PY -2516-05740 America/Asuncion QA +2517+05132 Asia/Qatar RE -2052+05528 Indian/Reunion RO +4426+02606 Europe/Bucharest RU +5443+02030 Europe/Kaliningrad RU +5545+03735 Europe/Moscow RU +5312+05009 Europe/Samara RU +5651+06036 Asia/Yekaterinburg RU +5500+07324 Asia/Omsk RU +5502+08255 Asia/Novosibirsk RU +5601+09250 Asia/Krasnoyarsk RU +5216+10420 Asia/Irkutsk RU +6200+12940 Asia/Yakutsk RU +4310+13156 Asia/Vladivostok RU +5934+15048 Asia/Magadan RU +5301+15839 Asia/Kamchatka RU +6445+17729 Asia/Anadyr RW -0157+03004 Africa/Kigali SA +2438+04643 Asia/Riyadh SB -0932+16012 Pacific/Guadalcanal SC -0440+05528 Indian/Mahe SD +1536+03232 Africa/Khartoum SE +5920+01803 Europe/Stockholm SG +0117+10351 Asia/Singapore SH -1555-00542 Atlantic/St_Helena SI +4603+01431 Europe/Ljubljana SJ +7800+01600 Arctic/Longyearbyen SJ +7059-00805 Atlantic/Jan_Mayen SK +4809+01707 Europe/Bratislava SL +0830-01315 Africa/Freetown SM +4355+01228 Europe/San_Marino SN +1440-01726 Africa/Dakar SO +0204+04522 Africa/Mogadishu SR +0550-05510 America/Paramaribo ST +0020+00644 Africa/Sao_Tome SV +1342-08912 America/El_Salvador SY +3330+03618 Asia/Damascus SZ -2618+03106 Africa/Mbabane TC +2128-07108 America/Grand_Turk TD +1207+01503 Africa/Ndjamena TF -492110+0701303 Indian/Kerguelen TG +0608+00113 Africa/Lome TH +1345+10031 Asia/Bangkok TJ +3835+06848 Asia/Dushanbe TK -0922-17114 Pacific/Fakaofo TM +3757+05823 Asia/Ashkhabad TN +3648+01011 Africa/Tunis TO -2110+17510 Pacific/Tongatapu TR +4101+02858 Europe/Istanbul TT +1039-06131 America/Port_of_Spain TV -0831+17913 Pacific/Funafuti TW +2503+12130 Asia/Taipei TZ -0648+03917 Africa/Dar_es_Salaam UA +5026+03031 Europe/Kiev UA +4457+03406 Europe/Simferopol UG +0019+03225 Africa/Kampala UM +1700-16830 Pacific/Johnston UM +2813-17722 Pacific/Midway UM +1917+16637 Pacific/Wake US +404251-0740023 America/New_York US +421953-0830245 America/Detroit US +381515-0854534 America/Louisville US +394606-0860929 America/Indianapolis US +382232-0862041 America/Indiana/Marengo US +411745-0863730 America/Indiana/Knox US +384452-0850402 America/Indiana/Vevay US +415100-0873900 America/Chicago US +450628-0873651 America/Menominee US +394421-1045903 America/Denver US +433649-1161209 America/Boise US +364708-1084111 America/Shiprock US +332654-1120424 America/Phoenix US +340308-1181434 America/Los_Angeles US +611305-1495401 America/Anchorage US +581807-1342511 America/Juneau US +593249-1394338 America/Yakutat US +643004-1652423 America/Nome US +515248-1763929 America/Adak US +211825-1575130 Pacific/Honolulu UY -3453-05611 America/Montevideo UZ +3940+06648 Asia/Samarkand UZ +4120+06918 Asia/Tashkent VA +4154+01227 Europe/Vatican VC +1309-06114 America/St_Vincent VE +1030-06656 America/Caracas VG +1827-06437 America/Tortola VI +1821-06456 America/St_Thomas VN +1045+10640 Asia/Saigon VU -1740+16825 Pacific/Efate WF -1318-17610 Pacific/Wallis WS -1350-17144 Pacific/Apia YE +1245+04512 Asia/Aden YT -1247+04514 Indian/Mayotte YU +4450+02030 Europe/Belgrade ZA -2615+02800 Africa/Johannesburg ZM -1525+02817 Africa/Lusaka ZW -1750+03103 Africa/Harare
