I suggest "first day of week" as an option available in RTM settings, for easier user customization. Same RTM already provide for date/time format, since it sounds more a user preference than a determination of formal/official country first day of week. Even I live in a country Sunday is 1st day of week, I prefer use Monday in any calendar picker.

In case of RTM Web, "first day of week" depends on the language set (e.g. UK English provides Monday as 1st day of week). In Android, you need to set, out of RTM app, your system to a country that matches your 1st day preferences. For instance, I am Brazilian, and if I want to keep Monday in RTM picker date (be it on Web or Android), I will need to use UK English instead of my mother language. Not ideal...