How to calculate (add) datetime values in MQL4?

MQL4 documentation declared datetime type to be internally represented as an amount of seconds since an agreed time-scale datum ( being 1970-01-01 00:00 ).

This said ^{( and polishing a bit the syntax compliance )}
the code
may read

oneYear = 60 * 60 * 24 * 365;   // yes, astronomers would kill me
                                // for not solving those seconds,
                                // that sum up until a leap year
                                // consumes 'em on Feb-29th day     :o)

another option
so as to manipulate
datetime in a bit more
comfortable manner, addressing
datetime's natural component is hacky, but worth: StringToTime

string TimeToString( datetime aDatetimeVALUE,
                     int aModeOfDISPLAY = TIME_DATE|TIME_MINUTES
                    )

Converting a value containing time in seconds elapsed since 01.01.1970 into a string of "yyyy.mm.dd hh:mi" format.

Here, one can simply add +1 to proper position of this intermediate format ( without a need to handle all the derived and influenced values as present in the struct MqlDateTime, where day_of_week and day_of_year are definitely not my favourite ones to re-calculate once moving +1 month etc.

aCurrentYEAR  = int(  StringSubstr( aDatetimeSTRING, 0, 4 ) );
aCurrentMONTH = int(  StringSubstr( aDatetimeSTRING, 5, 2 ) );
aCurrentDAY   = int(  StringSubstr( aDatetimeSTRING, 8, 2 ) );

aNextYEAR     = aCurrentYEAR  + 1;
aNextMONTH    = aCurrentMONTH + 1;

Finally

StringFormat( "%04d.%02d.%02d 00:00", aYearNUMBER, aMonthNUMBER, aDayNUMBER )

will do the re-assembly for calling another MQL4 standard function:

datetime StringToTime( string aDatetimeSTRING )

The function converts a string containing time or date in "yyyy.mm.dd [hh:mi]" format into datetime type.

Another approach may use a fully-decomposed datetime aritmetics by using

int aYE  = TimeYear(      aDatetimeVALUE );
int aMO  = TimeMonth(     aDatetimeVALUE );
int aDA  = TimeDay(       aDatetimeVALUE );
int aHO  = TimeHour(      aDatetimeVALUE );
int aMI  = TimeMinute(    aDatetimeVALUE );
int aDoW = TimeDayOfWeek( aDatetimeVALUE );
int aDoY = TimeDayOfYear( aDatetimeVALUE );

datetime aSameTimeNextYEAR = StructToTime( (MqlDateTime) { aYE + 1,
                                                           aMO,
                                                           aDA,
                                                           aHO,
                                                           aMI,
                                                           aDoW,
                                                           aDoY
                                                           }
                                           );