Just don't do it on the front-end. Never do it on the front-end.
You have to. User specifies thing should happen at 2PM, their time, then POSTs that data to your API or whatever. Well, what time are they actually talking about? You need to know.
You ask the user what time zone they're in? Better to have accurate data then guess.
Especially considering this is consistent with not identifying the device beyond headers sent etc. and you can't just use an IP address to map the timezone as it may be shared at the tower/exchange kilometres away (DSL) or hundreds of kilometres away (wireless/cell) and cross timezones.
The best strategy is to suggest the [timezone|local shop|city name|whatever] and prompt for confirmation.
Moment.js is a must-have.
If only we didn't need to add so much to our payload just to handle dates.